Well, after writing the thread title I wondered what defines this kind of game in the first place. Wikipedia lists them as "Vehicular combat games" (there are a few in there where the game isn't inherently a racing game like Rage and Auto Assault). What seems to primarily define that list is power ups and destructible opponents but I'm not so sure a game like Split Second is in the same vein as say, Mario Kart. I guess rubber-band AI would have a large role in defining this as well but that system is present in more than just these types of games. I guess it's up to you as to what defines a cart racer.
From what I've read Sonic All-Stars Racing Transformed is supposed to be really good (?) but I haven't played it, so I'll say Mario Kart 7. Anyway, I thought the responses to this will interesting because there hasn't been anything close to a standout this gen in this weird racing sub-genre.