So, based on a quick bit of research I did, GG Poker has a pretty solid player volume on average, and their rake is supposedly "very low" compared to other
poker sites. It'd be the ideal spot for anyone looking for high liquidity and an aggressive rakeback system.
pokerstars still has a decent amount of cash game traffic. Their rake is competitive, and if you're after stable software with years of market respect and a bigger variety of games, you'd be in the right place.
Just a step below those two, you've got
partypoker and Winamax. They have medium-sized player pools, some of the lowest rake rates out there, a lot of European players, active tables, and so on.
And this isn't just me guessing, eh? I actually did some digging through PokerScout, GameIntel, TwoPlusTwo, and even Reddit. The news sites are the usual suspects—PokerNews and CardsChat—where you can find a decent amount of discussion on this whole topic.