Unfortunately there are a lot of restrictions depending on, where you live. I live in Europe and has recently moved at least partly to 888 after playing for a year on PokerStars. PokerStars is way larger than anyone else, and for me it is also the best site for beginners.
This is because, they have a low cash game rake especially at 2NL, where a beginner should always start. If you want to play tournaments, Stars is also a good place to start, because they have a lot of 45-180 man SnGs at the low stakes, which is something, other sites can just not match. And importantly their software is also without competition.
However if you are already a winning player and have a
bankroll of maybe 300$, then 888 offer some advantages. Their tournament fee is lower, they have a lot of
freerolls, and competition at the cash tables is perhaps somewhat softer than on Stars. I say "perhaps", because for me personally I have not yet been able to see any benefits from this, since I have started with a pretty large downswing. That could still just be variance though, and/or getting used to the new software, so I am not giving up on 888 quite yet.
If you are based in the US, then I can not really advice you, because most of the sites, that are open to you, are not open to me. I can not play on Ignition or WSOP.com for instance, and with the recent turmoil I am not in the mood to try out ACR.
