Super Circuit, but it was still pretty good.
The most disappointing is 7, which had one of the best course designs but every other decision for the game did not bode well with me and ruined a lot of it for me.
Best --- Worst
Wii - Double Dash!! - DS - 7 - 64 - SNES - Super Circuit
Order I played them for reference (it seems to influence people's least favorite):
First --- most recent
64 - Double Dash - DS - Super Circuit - Wii - SNES - 7
I predict 8 will easily beat the Wii version as my favorite, though as long as they keep single player VS mode.