Depends on what you like??
Draft leagues are very popular but most of them have sooooo many people it's almost impossible to win some cash. CBS Sportsline has some good cash for winning you 10 man league. They use the same software as CNNSI and NFL.com. All have free leagues I think as well as pay leagues. Prices vary depending on payout of the league. ESPN and Yahoo have good live drafts but you can forget about winning any cash unless you get lucky or just DAMN good.
CDMsports.com has a real good selection of fantasy games and some good ones for free. They have a college game I plan on playing with over $5000 at stake. They probably have the widest variety of fantasy games.
Fanball.com has some decent games along with Smallworld.com. Sandbox software suck so forget them.
There are a bunch more but these are a few I have played and liked.
It depends on what you are looking for and how much you want to spend...entry fee... transaction fees...etc.
Good luck!!