Got anything to add? Remove? Let me know in the comments, I'll make sure to credit you!

LEG ARMORS

Nothing - Literally nothing gives you a strange thong. Available immediately, no armor bonuses but let's be honest: you're not here for protection.

Old Aristocrat Shoes - Same as nothing but you get dope* shoes and a chub. Go nuts, literally.
(*Shoes may not be dope)

Mushroom Legs - Shares the thong with Nothing but you get covered in mushrooms. Your fungass also grants you insanely high Immunity, Focus and Vitality!

Gold Waistwrap - Oh yeah, Gold Mask had the right idea 'til his gold ass t-posed to death lmao. But it's yours now, and it's incredibly revealing but covers just a teeny bit less than nothing. Comes with great Focus and Vitality + blinged out leg bracers and crotch cover. You gotta finish Corhyn's quest to get it though, and it's really weird so like maybe find a guide on it.

Champion Gaiters - Congrats you've graduated from thong to diaper, ya big baby. Covers up just a bit more butt than the previous, you also start with it if you pick Hero so that's a plus.

Rotten + Reg. Duelist Greaves - Okay, okay, it has minuscule cheek windows and they can KINDA clip when running, but generally not viable when it comes to total cheek-age. Comes in flamin' rot red too.

CHEST ARMORS

Prisoner Clothes - Grants minor ass visibility but flaps up while running so it kinda works. Comes free with Prisoner if you wanna have some glintstone cheeks.

Mushroom Body - PRIME. ASS. VISIBILITY. Displays your rump beautifully without distraction. Pairs well with the fungass.

Goldmask's Rags - Goldmask back at it again with showing off his hams. You can see them cheeks well.
Same issue with the waist wrap, it's locked behind that blind folded guy's stuff.

Fell Omen Cloak - You can just barely see ya buns peeking through, but you can see it better at an angle. Obtained later so not generally optimal for cheekage.

Leather Armor - Fashionable, good butt view, overall pretty slick. You do need to kick Patches' inferior ass though, so that may be a problem.

Eccentric's Armor - Very bad, no good butt viewing, just barely visible cheeks, shouldn't even really be here but just makes it.

Marionette Soldier Armor - Even worse than the previous one. I mean it's there if you squint. Also has anyone noticed the metal cage twist like fabric or rubber? It looks mad jank and once you see it you can't really un-see it.

Altered Cleanrot, Twin and Fingerprint Armor - I slapped these together because they all generally cover that tarnished tush but still leave an eensy weensy bit of skin. All of these must be altered though.

Altered Veteran's Armor - Probably the best heavy armor for exposing your ass. You gotta beat up lightning man for this one, which you'll probably be doing anyway on a non cheeked up run.

Altered Scaled and Altered Radahn's Lion Armor - Kinda show that rump, but like, they have this cringe leather skirt that sort of obscures the goods. Alright choice for heavy armor with a tad cheekitude though.

(Props to fonzie_GTA for this one!) Rotten + Reg. Duelist Cloak - Just a big ol' pair of snakes wrapping around your arms, excellent booty exposure.

(Thank you so much Evicatos!) Zamor Leg Wraps - Not only do you get DIVINE cheekage with this, you can show off those barking tarnished dogs too. Now in signature Lands Between filth-stained gray!

General Armor to avoid! - Any robes, cloaks, hoods, or armors with capes and long adornments. Most late game gear will most likely obscure the butt so look out for that.

SPECIAL CASES

Guilty Hood - Drapes down, hides your ass, not very cool. Plus it's supposed to like, feel itchy, and have a thorny branch collar, and generally not good.

Godskin Apostle Hood - Same as the guilty hood, drapes down and covers that fine piece of tarnished booty with some guy's face.

Navy + Crimson Hood - Again, same as before, covers butt. Nothing witty here.

Cleanrot Helmet - Okay, kinda different. The helmet has these little scarlet hair capey thingies. They're obviously bad because they cover up your butt. Not entirely, but it covers it pretty well. Go alter that thing and get 'em outta here.