I don't play much online and only really use PokerStars and Grosvenor. I would say that PokerStars was noticably tougher than Grosvenor.
GG Poker seems quite popular at the moment, though there was a bit of a cheating allegation recently. That centred around one player, who has been suspended.
Actually, the colour coding for Punters Lounge players indicates where they are in the league....this month red represented a player who could finish ahead of me,