This question has been asked many times. Mine always changes. I used to like Claritrans a whole lot, they are still up there. But lately ResearchTasks had some really great HITs that were pretty fun. Crowdsource is also stepping up significantly.
Worldsoyster... And, there are a few other requesters who have given me consistently good money over the years.
WorldsOyster is a curious choice. FireFox keeps telling me they are a known attack site, so I avoid them like the plague.
Mine has got to be AboutUs, but they've been gone for a few years. I could EASILY make $50 a day, if not more, by doing their stuff.