Probably Morrowind.

Properly distinct beast races, freedom to fuck up the world, really rewarding exploration, awesome scenery and concepts, great soundtrack, you can free slaves, and Argonians have never looked better overall.

Second place, probably Red Dead Redemption 2. Every single aspect of that game is outstanding. The pacing, the story, the characters, the combat, the exploration, the horse genitals, the music, the graphics, the massive scale of the world… Just truly a masterpiece of a game, and I think Rockstar will never surpass it.

Honorable mentions for Caves of Qud, Metal Gear Solid, Final Fantasy 7 (original AND remake/rebirth) and Starflight (mostly played on C64 but Genesis/Megadrive version is far better)