I feel like MK64 has that "dumb fun" factor that makes it pretty easy to return to on occasion, although I do have a few small nitpicks with it that later Mario Karts would iron out and refine such as the drifting and red shell pathfinding.
DKR is also really fun since you can stack items and race in other vehicles besides just karts, not to mention the story mode which is still something I wish Mario Kart would adopt