Tiers break down into who has spellcasting more than anything else due to spells being better than anything else in the game.
6: Skill based. Commoner, Expert, Samurai.
5: Mundane warrior. Barbarian, Fighter, Monk.
4: Partial casters. Adapt, Hexblade, Paladin, Ranger, Spelltheif.
3: Focused casters. Bard, Beguiler, Dread Necromancer, Martial Adapts, Warmage.
2: Full casters. Favored Soul, Psion, Sorcerer, Wu Jen.
1: Elitists. Artificer, Cleric, Druid, Wizard.
0: Gods. StP Erudite, Illthid Savant, Pun-Pun, Rocks fall & you die.