One of the "best" is clearly NitrogenSport.
It's not the one giving the best odds I think but it's a very secure one.
Everyone trust it, and they never had any problem.
I had trouble with some weird bets that didn't pay out and they really went fast at correcting it!!!
I recommend it.
This is probably the most important point. Theres no use in good odds if you cant ever cash out of the site or if they're a fraud... If a site is legit we need to spread the word about it to let everyone know (and also if a site isn't, we must warn the other users to avoid it).