Only having owned 3 of them and played 5 I can't say for sure, but MK64 is easily the worst of the 5. (and as MK7 tends to be high on a lot of peoples lists I doubt I would rate it below 64, so super circuit is the unknown entity)
I think most people's love of MK64 stems from a combination of nostalgia and local multiplayer. Back then most of us were between 5-15 years old and it was common to have friends over for gaming matches, most games seem 10* better when playing with 3 friends.
Double Dash was 7 years later, some of us were older and didn't have friends round as often, those still in the right age gap probably loved DD if they hadn't played MK64 before... not because 64 is better but because people don't like change.
Mario Kart Wii is another 5 years later and has online play... which is great but it's not the same as local MP.
MK64 has some great tracks (Sherbet Land, Jungle Parkway, Bowsers Castle, Banshee Boardwalk, Kalimari Desert) some unimaginative but none-the-less enjoyable tracks (Royal & Mario Raceway, Frappe Snowland) and some totally horrible tracks (Rainbow Road, Yoshi Valley, Wario Stadium) Which is just the same as most other Mario Kart games. The only thing it did better than later games is battle mode.
