Upwork, Guru, Worknhire have My trusted sites
I will go with Upwork....
Depends on what service you are offering/looking for. As others have stated, UpWork is great, and because of their size, it's easy to find a wide variety of freelancers and jobs.
One website that hasn't been mentioned on here yet is Toptal, which has the most rigorous screening process out of any freelance website (they claim to be more selective than Harvard and the Navy Seals). If you are an A-List freelancer, or you're looking for the best talent on your team, Toptal is the way to go.