To judge from the 70+ mods I have juggling around in OBMM (and the frequent use of Wrye Bash whenever I find a new one), I'd say Oblivion. I'd also say "everybody talks about the weather but nobody does anything about it", except I did do something about it, namely download Max Tael's weather mod.
I pity the poor fools who have to play Oblivion on consoles.