So the three largest poker sites right now are
GGPoker
PokerStars
PartyPoker
Last time I checked, PartyPoker had a rule specifically for Germans (just for me, yay!) that made you pay an additional ~20% on top of rake as an entry to every game, to... discourage gambling? It was very strange. I don't know if there's any player in the world good enough to make a profit under that rule. So that's that. Afaik, no other site does that.
GGPoker is the biggest $$$$ pile of $$$$ with $$$$-$$$$-$$$$ interface $$$$$$$$. It starts off not letting you choose where to install, then it went into a very slow installation with a progress bar that took longer than that of any other client, then when it was done (I was going to go to sleep after installing) it started another progress bar, and I have no idea how many it was in total, but perhaps 3* the length of the original one. None of them cared to inform me what was happening mind you. Then when you get there, it blasts obnoxious Christmas music without asking you first and has a ridiculously cluttered and obnoxious interface. It's like they were optimizing for me to hate it. It's also now the largest poker site. It has no sit&go's (????) (these are small on-demand tournaments and my default way to play) but does have cash games, hidden somewhere between great alternative formats like ALL-IN OR FOLD.
PokerStars is a mildly annoying, reasonable tasteful interface with very good customization. Also has some nonsense gambling formats and blackjack or whatever mixed in.
So the upshot of all that is that... if I ever play cash games, I should probably play them on GGPOker? The interface is so obnoxious that I just cannot believe there isn't at least a small positive correlation between people who prefer PokerStars and their skill. So you'd want to go where the weaker players are. And the rake is slightly lower as well.