Starcraft: Can play your entire life and never master it completely. Three races to learn and a game creation mode with multiplayer and co-op.
Sometimes I refer to my gaming dark ages where I was really disillusioned with games, disappointed with all my fav IP and stopped spending money on gaming completely for years.
What I was actually doing that whole time was playing Starcraft.
I agree that it's timeless, but I don't think I'd pick a game/series that only works as a zillion APM m/kb game to be the ONLY one. I think taking all things into consideration, #1 as a series is still Super Mario.
If you take Mario games, you leave only a few blind spots that need to be filled in elsewhere, but they're some massively huge blind spots. Starcraft, Halo, Zelda and Mario leave very little to be desired and are probably my top played IP of all time.