I've played and loved all of the Fallout games (that doesn't include the spinoffs like Tactics and BoS, I mean the real ones). The first one was just unlike anything else - at the time, it was THE definitive post-apocalypse game. The huge world where you had pretty much complete freedom was great. Fallout 2 was more of the same, but in the best way - it gave us even more stuff to do, however we wanted to. And then 3 brought the real innovation, bringing the world of Fallout into first-person 3D.
I've also played both of the Mass Effect games, and definitely plan on playing the third. The first game introduced a rich sci-fi setting with tons of interesting background info. The 3rd-person shooter combat wasn't perfect, but it was still fun. The sequel polished and streamlined the combat to near-perfection, and the story maintained the high standards of the original, while introducing some of the best characters in any game. Now I can hardly wait to get my hands on ME3.