SNES to N64 on a technical level.
But from a personal standpoint GameCube to the Wii is my favorite. Motion controls, traditional controls, and the games: Super Paper Mario, New Super Mario Bros, WiiSports, Twilight Princess, Metroid Prime Trilogy, Super Mario Galaxy 2 were phenomenal. Aside from the GBA, I don't think I've had as much enjoyment on a Nintendo platform than I did on the Wii.