I can only tell you my personal experience as I have only ever used odesk. It happened to be the first one that was recommended to me and I liked it, still like it and have had no reason to change. So to me, Odesk is the best.
My vote goes to both Odesk and Freelancer. I have been recommending those sites with my friends. But unfortunately, no matter how legit a freelance site is, freelancers must always be aware that there are scammers out there. And same thing goes with freelancers. So, we should always try to be careful when working on any projects.
freelancer.com is very very very bad. Actually if you have any problem with any buyer or provider they do not provide any support to you. I m in a dispute from last 2 months but still no response from freelancer.com. Odesk or rentacoder is much batter then freelancer.