I have only tried
pokerstars and
888 Poker, and among those two I prefer 888 for mainly two reasons:
1) In micro and low stakes MTTs PokerStars charge 20% more rake than any other
poker site.
2) The fields on PokerStars tend to be very large, which increase variance.
Apart from those two factors playing on PokerStars is great though. Excellent software and lots of different games to choose from. And there is also a solution to both these problems, which is to go in the SnG lobby and play the MTT SnGs. These are great tournaments for grinding, but often not quite as soft as the MTTs on
888 Poker, and during low volume hours only the lowest buyins and some of the turbos will actually fill.