After testing piles of fonts, sticking with Caecilia LT 65 Medium.
Tested Minion Pro, Arial, Fontin, Verdana, Droid, Charis, Times, Garamond, Georgia, Bakerville, Gentium and more.
I clearly prefer serif fonts since they are made for reading large amounts of text.
But most of the serif fonts seems to produce undulating or just varying blackening on the screen which I dislike. Caecilia in comparision is (at least in my eyes) very clear with a perfect distribution of darkness over the screen.