The prequel to Oblivion. Morrowind is definitely outdated in terms of graphics and whatnot, but the depth and the sheer vastness of Vvardenfell, in my opinion, dwarfs Oblivion. It's also significantly longer. Oblivion is about 60 hours more or less, Morrowind plus its expansions will rob you of about 200+ hours of your life.
I could also say World of Warcraft, but that one's too obvious.
This is probably me just nerding out, but Bethesda should do something Valve-esque, and re-release Morrowind, redone with the Oblivion engine, updating controls and graphics, but leaving most of the core game the same. (I hated the fast travel in Oblivion, idk why).
I think i went off topic enough.