it is pretty tough to say. i used to play limit and i thought that the juciest fish were on from about midnight to 3 am. i now play no limit and i feel like the best time to play is 8pm to 1am. others have said that the day nl games are pretty passive and soft. one thing i'd say for sure is that the craziest fish are playing on friday and saturday nights. it is very rare that i don't play from 8pm to 4am on friday and saturday.
as for full tilt vs stars vs the other sites i don't know how much this differs from site to site but if you want to see a good site that tracks the real money traffic on all the major sites check out:
www.pokersitescout.com
they update at various times throughout the day and have weekly averages and the likes so you could figure out a lot from there if you were to monitor it for a few days.