if you could only ever own and experience ONE Bethesda game
http://www.youtube.com/watch?v=lzLrYw4dowA&feature=relatedI'm kidding, of course. Assuming we can use (or at least make) modifications to our heart's content, Morrowind would be the best choice. It's still unrivalled in the amount of mind-numbing little details and easter eggs the developers tossed in, and to this day many people are still finding new things throughout the vanilla game. The various cultures and historical background never get old and help keep the world alive, both in the game and in the player's own fantasies. The generous use of divine metaphysics and Vivec's CHIMDUDE ramblings are enough to keep even the most diehard lore fanatic frying their brain cells out for years. Then
with mods, there's really the assurance that you'll never get bored!
If we can't use mods however, I'd say Daggerfall. It's just the best in entertainment value and roleplaying in its design principles and foundation. :foodndrink: