chapter 6 - because you can go back and do all the stuff you didn't do (with a bit more expereince) and there are new things to do.

Gotta agree with all that liked Two, though Six is still my fave. Agree also that, from a roleplaying point of view, you really should do the shadow thieves quests and get to Spellhold and rescue poor li'l Imoen. But, you gotta help Nalia and do the Copper Coronet to get the cash.