Super Mario Kart: Not one of my particular favorites (it just doesn't seem very exciting), but it's still pretty nice, and difficult without borders.
Mario Kart 64: I don't like this one too much. The visuals are great, but the track is just one long border-guarded straightway with no challenge at all. Then again, I haven't played this one in ages so I might be remembering it wrong.
Mario Kart Super Circuit: It's similar to the one from SMK, but I like this one a bit better because of the added elements like the bouncy borders and falling stars.
Mario Kart Double Dash: This one's my favorite. It has several nice elements like the twisting track, and it's quite a challenge as well.
Mario Kart DS: It's decent, but to me it seems like they focused more on just making it look cool then the course itself.