I would say go with a site that Is trust worthy, a site that has a name for Itself online.
Poker-Stars
Full Tilt
party poker
Bovada/
bodog
Titan Poker
888 Poker
william hill Poker
Full Flush
America's Card Room
Black Chip Poker
Now there are other sites that are questionable, since i've never cashout from "these" other sites, I'd rather not name them but the play on those other sites are alot softer than the play on the sites I've named above.
The Issue Is this, If you go with the known sites, there games tend to hold acouple thousand players In them assuming you play only tourneys, thus you would need to be winning flips and since the games are about GII good and running good you're more likely to end up with a bad beat than end up winning the entire game. That Is the Issue with the bigger sites, they hold bigger fields. Yet they also have bigger pay days. Now the smaller sites have smaller fields, yet the pay Is also smaller, and that's the problem with the smaller sites.