Depends on what type of player you are since different books cater to different players.
If you're a small bettor interested in things like free payouts, easy deposits:
BetJamaica
Bodog
Sportsbook.com
5Dimes
BetPhoenix
If you're a professional bettor looking to win a lot and not get limited:
Matchbook (exchange)
BetFair (exchange, no US)
Pinnacle (no US)
The Greek
Bookmaker
I'll also make mention of WagerStreet. They have some lines that are a good .20 different (better) than every other line on the board, these come up every other day. If you bet these you'll make a killing. Probably the easiest book on the net to beat.