Gotta be Mario Kart Wii for me - bikes are ace, control is perfect and love the levels, online play.
MK64 was good, but too problematic with collision detection - wasn't too fond of it, but loved the circuits.
DS was good, have fond memories of it, but haven't played it for years.
MK7 is pretty decent, getting back into it recently.
From what I've played of 8, it's bloody brilliant. Easily up there with Wii.