I probably sank an equal amount of time into SMK and MK64, both alone and with friends back in the day. I always thought they felt unique enough to stand by themselves, though of course everyone would have some things they wished were the same, that one had that they wished the other did etc.
It doesn't feel quite the same to drive, but it was tight in it's own way. Mastering the mini-turbos (before they oversimplified them) and other techniques made time trial highly addicting. 2 player GP with the CPU was where most of my time was spent - instead of a lone cup, we'd tally results from all 4 cups to determine the winner. House rules like that, and which bugged-out shortcuts you can attempt, or as someone else mentioned, just teaming up to take out the cheating AI. We made our own games within the game cause it was just fun. I feel like modern kart might be more refined, but it's maybe less nuanced. It had it's quirks but a lot of it felt intentional and ultimately ended up making it more fun. Even busted speed in 3P DK Jungle Parkway!
Battle mode in 4 player was a great improvement over 2 players just by default. But the larger arenas, lack of feather, the karts just generally being less snappy - we never had moments quite as exciting as SMK's best.
I'll happily argue that nothing in the later games beats the feel of flinging a green shell while turning in the original SMK, watching it arch around a barrier and WHACK right on target. They're much much too slow and basic in their movement in modern kart.
That way you can make the game your own is gives MK64 longevity over Diddy Kong Racing though. They both run like dogs now, but MK64 is still fun. When i first got DKR, sure, it looked much better and adventure mode is awesome! But by the time i finished that bastard Wizpig the 2nd time? Never wanted to play it again. I feel sorry for any actual kids who picked up that game. And the multiplayer was just plain slow and boring compared to MK64.
