According to
Thief Training Reference - MapleWikiChief Bandit Leveling AreasYou may want to consult these sections based on what skill you chose to max first. (Main Attack Skill-wise)
Assaulter FirstExp Areas85~100 Forgotten Path Of Time<3> (Death Teddy)
85~120 Forbidden Time (Grim Phantom Watch)
90~110 Unbalanced Time (Dual Ghost Pirates)
95~110 Lost Time (Master Death Teddy)
105~120 Twisted Time (Gigantic Spirit Viking)
Money Areas70~85 The Forest Of Gollem (Mixed Gollem)
70~120 Cursed Sanctuary (Jr. Balrog)
Rare Drop; Boss Monster, spawns at large time intervals.80~90 Dangerous Cliff (Dark Jr. Yeti / Dark Yeti / Dark Yeti & Pepe)
80~90 Cave Of Trial II-III (Cerebes / Bain)
Rare Drop85~100 Forgotten Path Of Time<3> (Death Teddy)
95~120 Forgotten Path of Time <4> (Phantom Watch)
All Around Areas70~80 Forest of Dead Trees I-IV (Coolie Zombie)
75~85 Ice Valley II (Yeti & Pepe / Yeti)
Band of Thieves FirstExp Areas70~75 Garden of Green / Red / Yellow (Jr. Cellion / Lioner / Grupin)
Party map, don't bother soloing85~120 Forbidden Time (Grim Phantom Watch)
95~110 Lost Time (Master Death Teddy)
105~120 Twisted Time (Gigantic Spirit Viking)
Money Areas70~85 Sanctuary Entrance II-IV (Tauromacis / Cold Eyes / Taurospear)
70~120 Cursed Sanctuary (Jr. Balrog)
Rare Drop; Boss Monster, spawns at large time intervals.95~120 Forgotten Path of Time <4> (Phantom Watch)
All Around Areas70~80 Forest of Dead Trees I-IV (Coolie Zombie)
70~80 Watch Out for Icy Path I (Jr. Yeti)
70~80 Watch Out For Icy Path II (Jr. Yeti /Jr. Yeti (Transformed) / Hector)
Meso Explosion FirstExp Areas75~100 Deep Sea Gorge II (Goby / Bombing Fish House)
If you're rich.95~110 Lost Time (Master Death Teddy)
85~120 Forbidden Time (Grim Phantom Watch)
105~120 Twisted Time (Gigantic Spirit Viking)
Money Areas70~120 Cursed Sanctuary (Jr. Balrog)
Rare Drop; Boss Monster, spawns at large time intervals.95~120 Forgotten Path of Time <4> (Phantom Watch)
Party Map; don't bother soloing.All Around Areas70~80 Forest of Dead Trees I-IV (Coolie Zombie)
70~80 Watch Out for Icy Path I (Jr. Yeti)
70~80 Watch Out For Icy Path II (Jr. Yeti /Jr. Yeti (Transformed) / Hector)
70~85 The Forest Of Gollem (Mixed Gollem)
75~90 The Grave Of A Wrecked Ship (Goby / Bombing Fish House / Shark / Cold Shark)
Party Map; don't bother soloing. If you're rich.