Like Amasius, I have to say many of my favorite RPGs are single outings like PS:T. Troika's titles or The Witcher or else games followed by less compelling sequels.

But I really have no trouble voting for a series; had to go with Might and Magic. I have no doubt that a lot of it's nostalgia, but when I think of all the hours of exploration, char dev and combat, it's unequaled in keeping me playing.

Of the other series, I can honestly say either I haven't played them-(too late for the party on the Ultimas, etc)-only played one to completion(Gothic, BG1, NWN;Sou and NWN2 OC))- or haven't played enough to judge. And while I've played all the Bioware games, I just can't say they are my favorites, even though they've been quite fun at times. After awhile they just wear out for me. Not M&M, though.