So, having thought about it some, I’m tempted to say: None. I guess that’s why I can’t keep myself from tinkering with homebrew systems that I never finish.
Taking another tack at it: My favorite systems right now are classic D&D and classic Traveller. (Tho’ I borrow the nd6 ≤ ability score bit from TFT for cD&D. & something vaguely similar for cT as well.)
There are several games that i like on paper, but I haven’t played enough to say in practice: Risus, Prince Valiant, Dungeon Squad, the Pool, Pendragon, Wushu.
And a few that I think have at least one very good idea that I’ve love to incorporate in my “perfect system” (if I still believed I could create it): the Window, Dying Earth, Lejendary Adventures.