This list is in order from best to worst, this includes items in the same tier.

S Tier
Lighter: Chance to light enemies on fire on hit- Once an enemy is on fire they are slowed and take 3 hits of damage over a couple seconds, I'd estimate the damage is equivalent to 3-4 strikes from the shovel for each tick of fire damage so about 9-12 shovel hits total. The only downsides to this item is it has limited opportunities for use against the conductor. and attacks will cancel out the fire damage if they occur on the same frame as the fire damage. generally, you should let the fire damage run its course before attacking again.

Acid: Sprays acid out of your watering can- Effectively replaces your shovel as your primary weapon, I'd guess it's about double damage, range and attack speed of your shovel. The major downside to this item is that it is the last one you buy from the shop and it is the only useful one, buying everything else increases the pool of items you choose after each boss making it more difficult to get the setup you want. You got 4 item slots, only 5/6 items are really useful and there are 11 items total if you buy everything, see the problem? The watering can also cancels any fire effects if you happen to also have the lighter on you, so again, I'd recommend letting the fire burn out on an enemy before continuing to attack.

Edit: As mentioned below in the tips section, a comment below states it's possible to skip all items except for the acid, if this is true ignore my comment about the item pool and just get hats and acid until you're finished with limitless line.

A Tier
Coffee: Makes you move faster- It is far easier to dodge and get into striking range with the increased speed, though it takes some getting used to. You may run into projectiles and enemies you wouldn't have otherwise.

Riot Shield: Chance to negate all damage when you're hit- This doesn't negate hit stun or knock back, but the chance for it to take effect is decent enough to be noticeable. You might get better mileage out of one of the 2 healing items but both of those are based on RNG as well. An update has also allowed it to block poison damage making this even better of a choice compared to healing items.

B Tier
Plastic Fangs: Chance to heal a little bit when an enemy is killed- Heals about one heart every time it procs, The riot shield seems to activate at about the same rate as the fangs and it has the potential to negate more damage than the fangs provide.

Care Package: Chance to heal a lot on room clear- it heals about 4-5 hearts every time it procs, If you don't have any of the above as an option I'd pick this up just to have an extra healing option besides heart pickups and the lollipop nurse.

C Tier
Armed Drone: Flies at and stabs enemies whenever it feels like it- Mediocre, It doesn't attack frequently enough, only does about a shovels hit of damage and sometimes will miss completely. Allows you to hide and dodge attacks from projectile enemies while it hits them for you, but it's gonna take a while. It has knockback on its attacks so it might save you if you're getting charged or knock an enemy into you.

Sawblade: Big ol' blade circles around you damaging enemies it runs into- Kinda janky, messes with enemies post hit invincibility, kinda just gets in the way, it sometimes gets a hit when you're unable to or when you miss, but it doesn't spin around you fast enough to be consistent. I'm not sure if it's able to block projectiles, you're better off with Coffee or the Riot Shield even if it does.

D Tier
Fairy Wings: Tripping (Dodging) has a longer range- You shouldn't be dodging all that much anyway, sure, you're briefly invincible, but you're locking yourself in one direction, limiting your ability to counter attack and you are vulnerable when you are getting up. from my experience fairy wings put you even farther from a good position and gets you hit more than it helps you...Stick with Coffee.

F Tier
Bull Horns: Damages enemies when you trip into them- and you take damage too, hurray! Does about 2-3 shovel hits worth of damage, not worth it, but it's kinda fun?

Explosive keg: Chance for enemies to explode when they're killed- and you explode too, hurray! It might have some synergy with armed drone but I haven't bothered to try it. On its own It'll clear a room but Unless you dodge backwards or run away after every enemy you kill you're going to get caught in the blast and take about 3 hearts of damage. Definitely not worth it, and it's not fun. It's funny, but not fun.

Big ol' Pig: in the first loop you can kill him before his second charge if you attack frequently enough. He's effectively just a larger version of any charging enemy. His charge is slowed down if he's on fire.

Elite variants

• Aggro (Red): Faster pig, dodge earlier
  
• Steel Plated (Gray): Beefier pig, attack more
  
• Poison (Purple): Poisony pig, don't get hit
  
• Giant (Green): Big big pig, get out of the way more

Red cat: Spins around and bounces off walls like a billiard ball, spits out worms underneath him, repeat. It's not worth attacking him while spinning unless you have the watering can for more range, hide in the lower part of the room around the boxes if you have to, but stay close enough so you can get the maximum damage in when he stops spinning. If you light him on fire and then light one of the worms on fire the worms can relight him and he'll take 2 rounds of fire damage.

Elite variants

Elite worms may spawn after loop 1
• Aggro (Red): Faster while spinning, faster worm spitting, less idle time to get hits in, don't be greedy
  
• Steel Plated (Gray): Fire will help chip his health away if you have a lighter otherwise just wack dat cat.
  
• Poison (Purple): Don't get caught in a corner, otherwise multiple hits and the poison will take you out pretty quick
  
• Giant (Green): Big cat, Stay in safe spots, he takes up about a quarter of the movable area

Nuclear Deer: Charges, makes rabbits pop up from the darker grass spots, shoots some goop at you, repeat. Rabbits can relight him on fire similarly to the worms with the cat boss. Attack him after his charge as much as possible just like the pig.

Elite variants

Elite Rabbits may spawn after loop 1
• Aggro (Red): Faster charge, faster rabbit spawn, faster goop projectile speed, less time to get attacks in, blah blah blah.
  
• Steel Plated (Gray): Same as cat, light em' up
  
• Poison (Purple): The goop projectile causes poison as well, dodge it yo
  
• Giant (Green): You'll have a lot less room to maneuver especially with rabbits bouncing at you

Onion man: Charges and stabs, throws shurikens 3 times, Charges and stabs again, shoots horizontally across the screen 3 times repeat. Generally as long as you stay moving he can't hit you, move anywhere away from him during his stab, Move up and down during his horizontal slashes and move diagonally towards him as he's throwing a shuriken to get a hit or two in.

You can attack twice or more if you're quick enough after every stab attempt or shuriken throw, After the 3rd Shuriken throw he is idle for longer than normal, you may be able to get 5 hits or more if you're fast enough. The lighter is only useful here after his 3rd shuriken throw as he puts out the fire when he teleports but he's idle long enough after his third shuriken throw to have fire damage him 1 to 3 times. You may also be able to light him on fire if you are on fire and get near him, but it's risky.

Elite variants

• Aggro (Red): Faster attacks, less idle time, don't be greedy
  
• Steel Plated (Gray): Annoying, since you can't really use fire here this becomes a slog, Acid makes this a lot faster if you have it.
  
• Poison (Purple): Stay moving, If you're low on health and worried about the poison just focus on dodging the shurikens rather than using them as opportunities to get an extra hit in.
  
• Giant (Green): The only issue here is the larger shurikens, avoid going into corners or into the boxes at the bottom as he's about to throw them.

Spooky zombie turnip: He will appear randomly before other bosses, I've only seen him charge and stab similar to onion man and trip towards you, the lighter is amazing here. get a couple hits in after his stab then wait for him to trip towards you before attacking again. He's not too difficult he just does a lot of damage. He counts as a boss so you get an extra bosses worth of tickets from him. I haven't seen him with elite variants but he may have them, Steel plated and/or aggro would make him very very difficult, but perhaps he doesn't have elite variants because he'd be too difficult considering the damage he puts out.

Charging: Pigs and deer, don't dodge if you don't need too and be aware that deer take a lot of hits regardless of elite status at later loops, don't get greedy.

Projectile: Sheep and zombie donuts, sheep shoot 4 projectiles at once, one shot is diagonal in 4 directions NE, SE, SW, NW on a Compass, the other is North, East, South and West, very annoying, especial in the aggro variant as they shoot faster, attack when you're aware of the next shot pattern. Donuts shoot directly at you, also very annoying with the aggro variant use cover to get in close if possible or just zig zag closer in between shots.

Grabby: Cupcake, he can mess your positioning up, and his aggro variant pulls you towards him faster, don't prioritize him if you don't have to but get rid of him if you have the opportunity.

Slimy: Slime, a helpful enemy, hit him back towards other enemies and they'll take damage from his slime, donuts do not take slime damage as they are floating.

Bouncing: Popcorn (I think) treat them like smaller red cats, they're very useful when lit on fire to bounce back into other enemies and light them on fire as well.

Hopping: Wabbits, only a real issue when they're an aggro variant, they will hop at you relentlessly, attack them before they hop you, mash x and backpeddle if possible, dodge if needed.

1. Dodge only when necessary, it limits your ability to counterattack and may put you in a bad spot, I generally will only dodge Red Cat if Im in a bad spot but occasionally I'll dodge charging and projectile enemies too.

2. You attack faster when you're not moving, if you have any enemy against the wall just mash your attack button, though larger enemies may bounce back off the wall and into you.

3. Stay close to the entrance of a room, It's often better to let charging and bouncing enemies come to you then to dodge around them and attack. Try to avoid getting every enemy after you and make your way into the room slowly, killing one at a time.

4. Knock flaming enemies into another to spread the fire, or get enemies to get closer to flaming enemies by getting closer to them, this is a great way to deal with projectile enemies you don't want to mess with.

5. Buy the hats, skip the items, Acid is great, but filling your item pool with useless stuff isn't worth it. You're better off getting Lighter, Riot Shield, Fangs and Coffee most of the time, than getting Acid and 3 worthless other items. For now at least, maybe a future update will improve or replace the items.

Edit: According to Xureman in the comments below, you can skip through items by buying multiple hats and entering and exiting limitless line until you get the Acid, I haven't tested this, but, if this is true, definitely do that, Only buy the acid and hats until you get everything you wanted out of limitless line.

6. Don't waste tickets on daily challenges, Unless you want to be on the leaderboard, the daily challenges are just regular runs but with all items unlocked and a ranking system, go for it if you want to try all the other unlockable items but if you're grinding for hats, stick with the regular runs and the base items.

7. Get all the health, more health, less ded, ya feel me?

Here's a guide by the devs follow it if you're missing any.
https://steamproxy.net/sharedfiles/filedetails/?id=2463280158

Thank you for reading, hope I helped you get all the hats you want, if I've made any mistakes feel free to correct me in the comments and I'll edit the guide when I get a chance.