The Fallout series, but only from Fallout 3 onward. Not that I dislike the original two (it's Brotherhood of Steel I dislike), except that they are both incredibly glitchy, buggy, and have turn-based combat that literally makes me want to tear my hair out by the roots. I prefer running up to my enemy, pressing a button, and watching their organs splatter across the nearest surface instead of, "select weapon, select attack, select target, BAM! They're dead!"
And, of course, the Elder Scrolls series. Morrowind, if I had to choose just one, is my favorite game of all time.
...actually, I may just be a Bethesda fanboy. *moment of Fridge Horror*