I'm torn between MK64 and MK8. If I ever feel like playing a Mario Kart game then its one of these two. Battle Mode means nothing to me so I base my opinions on just the regular game in both single and multiplayer.
I think MK64 just edges this one. MK8 just has so much extra stuff over a regular Mario Kart, it plays really well but MK64 has my favourite tracks and I actually prefer the rubberband AI in GP mode over having the cpus use all available weapons regularly. I also prefer the weapon set a bit in MK64 but red and blue shell tracking has definitely improved in later games. I prefered the old drift boost too and generally thought that winning in MK64 felt like it required more track practice and skill but the flip side is that the weapons are a little more of an afterthought than a central mechanic, no real need for a horn to kill blue shells in this game because they aren't much of a threat.
To rank them I think it would go:
1. Mario Kart 64
2. Mario Kart 8, the first time I enjoyed online play on a Nintendo system.
3. Mario Kart 7
4. Mario Kart DS, I wish the mission mode was a series staple
5. Mario Kart Wii
That's the good ones and now:
6. Mario Kart Super Circuit, still very very nostalgic for me.
7. Mario Kart Double Dash, lots of problems with this one
8. Super Mario Kart, easily the worst but forgivable due to being the first and pretty ambitious given the constraints of SNES hardware.
I like all of them though.