[Lv1-2] Intro/tutorial map (Dragonwatch keep & Dragonwatch battlefield)
You'll usually always start at the intro map so complete the quest that will be given in there till you defeat the warfiend, and that it'll prolly get you to lv3 + there will be a shop for starting items and drops for your gears in the long run.
[Lv3-5] Greenguard (Greenguard forest)
After you complete the intro/tutorial map, you will be teleported to battleon, this time find the gate that will lead you to greenguard and start doing the questlines there, new gears and craftable items with decent stats can be attained there via farming for the parts of the specific item.
[Lv6-7] Greenguard (Heartwood forest)
Doing the main questline is always a 'first priority' in aq3d so do the main quests always, craftable items in this area is very important in your way from mid-level to move up, so get every possible monster item drops in here and craft them in the stone face npc for the gears.
[Lv 8-9] Greenguard (Livingstone caverns) option1
Gears in this area has an important stats you might need for you to be able to solo the next map, get the green gears in here, and as usual the craft ingredients required to get those are from the mobs inside the area.
[Lv8-9] Greenguard (Barrowpoint dungeons) option2
The dungeons in the barrowpoint is important for you to level up two consecutive levels, there are decent gears that are craftable from different dungeon you will encounter, make sure to try all 3 dungeons for more decent exp. [barrowpoint dungeon-take pattern: middle, right side, left side]
[Lv9-10] Greenguard >Livingstone caverns (Livingstone alcove dungeons)
The dungeon in this area gives you huge exp for you to get up to lvl10 so do the quests in here and get the gears in the craft shop of this dungeon, it is important as they are in 'blue-quality', make sure to get the miners hat and shovel or pickaxe for decent gears along the way.
[Lv11-12] Doomwood (Doomwood forest)
The classic map for lvl10's+, kill skeletons and shadow wolfs here because they are the only thing needed for the questline, and get the gears in the npc's craft shop, this is where it starts getting pretty tiring because of the repititive kills that youll probably need to do to progress the questline, but hold on tight cus you will be rewarded with pretty decent gears from the dungeons and an exp to get near the exp cap.
[Lv12-13] Doomwood (Bone cliffs)
In here youll be battling skeletons again for the questline, and youll meet new mob- the gargoyles, you will be killing it too to procced into the next map, also the dungeon to craft the current strongest weapon in the game can be found in here and that you need to complete 3 different dungeons to get it, gears that can be found in this map will help you a lot than other maps can offer you, so try to explore the area and find the npc's that will give you the gears.
[Lv14-15] Doomwood (Shadowskull bridge) option1
This is the last-push type of area, you will need to clear waves of skeleton groups consecutively with other players to get exp's from them, and at the same time completing the final questline, there might be some helpers you will encounter in the area to help you grind the exp for the last two levels.
[Lv14-15] Doomwood >Shadowskull bridge (Shadowskull tower dungeon) option2
It is not advisable to lvl yourself up in this area as lv15's just do the dungeon in here to get the final gears and to hunt for specific gears, but as this dungeon's mob gives huge exp, this might help you boost your experience to 15 and at the same time to ready your gears as soon as you reach the cap.
[Lv15-16] Ashfall ->Dragonslayer camp ->LowerFirefields ->UpperFirefields (Ashfall pt. 1)
This is the first part of ashfall where it takes place on dragonslayer's camp, there are hidden dungeons in here if you already completed the questline find the three portal books for the Scale slayer gear, check the wiki on how to find and get those for more info.
[Lv16-17] Ashfall ->TheSlabs ->BrimstonRuins ->MagmaMines ->TowerPass (Ashfall pt. 2)
You can get new gears, in magma mines dungeon, it will be better, finishing the storyline here gives you huge exp so be sure to do that as well.
[Lv17-18] Darkovia (Darkhurst)
Do Shadowslayer W's questline and Boog tavern's too untill you reach Lv18 procceed to the next area, a quest should navigate you towards the gate which will port you on the 2nd part of Darkovia which is darkovia forest.
[Lv18-19] Darkovia (Darkovia Forest)
Same as what you did in darkhurst: finish the story questline of the red hooded npc and craft her Lv18 gears to get decent stats for your toon.
[Lv19-20] Ashfall >MountAshfall Camp (Ashfall pt. 3)
Finish the story dungeon and procceed doing the daily quest for Dragon Trophy which is needed to craft your Lv20 gears, Talyn for DEF build and Berserker for ATK build. You will need a party when doing Akriloth for your Daily Quest' so make sure to bring some ppl in the area or call out some players in your friend list/guild.
[Lv20-21] Mysterious Islands (Lolosia)
The amount of Daily quest you can get in this area will really help you a lot, You can also get the pirate class in here so its highly recommended to do the daily quest here everyday, in-order to unlock it you will need to complete the storyline quests first.
[Lv21-25] Darkovia >Darkhurst (Vampire Castle)
The game has been in a long level increase stop on these stages so the 4-level increase was kinda acceptable, It has been almost a year and the level cap was still not increased hence why the lv21-25. Complete the storyline here and get the Bloodknight set, by doing so you should also get tons of exp closer to 25 and the best-in-stat of the current level, In-order to cope with the huge exp requirements dont forget to do all of the availble daily quests in the game, The aq3d wiki should help for the location of the quests so google it.