MK64 has aged terribly, my friends made me play it for a few hours recently and it was abysmal.
DD is great. I love the two character system, as switching gives an extra layer of depth to gameplay. The fact there are some really great items that never made a return to the series makes DD a blast to go back to as well.
MK8 is great too, but it almost feels too polished. I liked some of the chaos that DD brings, which is why I still say DD is the best in the series. MK8D did add a great online battle mode though, which has become my preferred way to play the newest game, not racing.
Anyone ever play the arcade games? Dave and Busters near me has 4 units linked together and I really enjoy it. I wish they would port it or make it DLC or something, it's a shame so many people have not played it.