Single player - F-Zero GX. The skill cap in this game is astronomical - mastering it akin to mastering a musical instrument. There's nothing else quite like it.
Multiplayer - Smash Ultimate. The amount of unique character matchups and interactions is so massive that there's always something new to see.
I would probably say morrowind with all the community mods and modern engine.
Skyrim with mods, honestly. I feel like there's a lot of potential there
If I was never introduced to today's graphics, easily Morrowind. I would still pick Morrowind over Skyrim.
You could readjust pretty easily within the span of the rest of your life.
Path of Exile.
Could probably be happy with Hades
Although if Halo MCC count as one game...
At this point, I think I might have to go for monster hunter. Or Dragon's Dogma. Just some kind of super deep action RPG would do it.
The Binding of Isaac: Repentance.
Pull up a chair and let me tell you about a little game called Nethack.
Doom/Doom II. Already been playing it for 3/4 of my life, and the modding community still keeps it fresh so even if the core game gets old, there's still things to do with it.