i would say Fallout 3...but i'm not..i'm going to say Oblivion.
why is Oblivion my choice over Fallout 3? one word...scenery.
Oblivion has the scenery and about 1000 more places to discover and things to do than Fallout 3, and about half-way through getting everything for Fallout 3...it's all the same, oh look..i'm in the wastes again...oh look another raider..oh..now a deathclaw..and supermutants! yay!
Oblivion, you never know what creature you'll run into, or person..random people with unmarked side-quests are always fun to run into.
Plus..the scenery is astounding ((have i mentioned the scenery yet?))
oh and also, the scenery..
....plus magic with swords
*EDIT*
my first and only account is level 24 or 25, i forget which, and i'm 79:43:52 into my play-time
for those of you who didn't figure that time out..it's 79 hours, 43 minutes, and 52 seconds...i enjoy walking around that world a lot..
*EDIT*