Mario Kart Wii had the most stunning graphics and effects out of all the Mario Karts, but in my mind it ties with the N64 one because that Rainbow Road has probably the best music, and it gives me nostalgia.
Honestly I can't figure out how to unlock new cups in Double Dash, so I have to go with Mario Kart Wii. Do you know how many times I've fallen off??? And I love the Star Bits floating around.