I rarely play more than one game at a time, since I don't devote enough time to them...
.. having said that, the Mass Effect Trilogy has seen me play through each of those games at least three times (well, minus the third.. on my first playthrough of my second character currently) as I have three characters, and then at least one more playthrough on top of the initial, for the added difficulty/additional levelling/achievement lol that comes with it.
i realize the only game I ever talk about is ME, but the trilogy's
that damned good.