Oblivion. It came out at the perfect time in my life.
Skyrim certainly moved the series forward in a few notable ways (aesthetics and combat in particular). But it also dropped the ball in too many ways; they completely stripped the soul away from two of my favourite things about Oblivion: potion crafting and spellcrafting. More than that, it completely fucked up guilds. They were both severely derivative and wholly worse than in their predecessor. The civil war stuff was also one of the most underwhelming quest chains in the series.
This is my list (I've finished every game, with well over 60-80 hours in each):
- Elder Scrolls IV: Oblivion
- Fallout: New Vegas (not technically a Bethesda RPG though)
- Elder Scrolls V: Skyrim
- Fallout 3