Back on topic for the OP, Alter Self can potentially break the game, and that's only at level 3. Tack it onto an Otherworldly level 5 wizard and BAM, you've got a Dwarven Ancestor that can cast spells and have higher than 30 AC at level 5, though that isn't core.
IMHO, once Polymorph comes into the picture it's game over. A wizard's options are virtually endless. He ignores environment-based challenges (traps, geography, etc) and he can fight better than a fighter. He also has access to spells that eliminate much of a need for being a sociable person. Writing an Illusory Script on his shirt that suggests everybody "be my bestest friend in the whole wide world" can make social encounters obsolete. He can scout reliability with clairaudience/clairvoyance.
For this reason, I think the true break down level is level 7, in the case of wizards.
In the case of druids, it's level 1. Animal Companion for combat, spells for BFC, summons/speak with animals for scouting. They aren't as good at social encounters, though, and can't quite ignore environmental threats until later. However, they're already stomping on the toes of the fighters and rogues, and can heal to boot. If you don't think that's good enough, by level 7 they roll up the game and light it on fire with strong Wildshapes.
Clerics don't quite have the versatility of either class above. However, at level 1 they are already stepping on the fighter's toes, and by level 5 they are stomping on them, and at level 7 they literally crush the fighter's foot. They don't have the strong scouting abilities of the wizard, but can pass okay at it by level 7. Environmental hazards must be dealt with in a pretty imaginative fashion, but they do have Summon Monster and Air Walk at level 7. I suppose for the cleric, the true breakdown point is level 7.
Just looking at the above, in my opinion the true, absolute break down point is level 7.
