Well, I've played DD, and here's my MK order, best to worst.
1. Mario Kart 64
2. Mario Kart: Double Dash!!
3. Mario Kart: Super Circuit
4. Super Mario Kart
First, lemme say that Double Dash is great. I've played it for many hours even though it's only been a week since it was released. It's very, very well done, but it certainly lacked in track design, it didn't have the "Quick Run" like Super Circuit did - the Balloon Battle mode wasn't half as good as 64, either. I liked the new Shine Steal mode addition a lot though - I also liked the variety of Special Items - one per character. I really did enjoy all the unlockables, ESPECIALLY the new Wario track. Babies Mario and Luigi had HORRIBLE voices, but you get used to that...I'd rate the music at about 7/10 - it's good, but tends to use the same theme a lot, and the synth could be better - it also takes time to get used to the whistling in EVERY SINGLE SONG. The credits theme is a nice mix of that main Double Dash theme and the credits theme of 64. It's hard to say which is better between 64 and Double Dash, because they both lack in some areas, and yet are both SO great in many areas.
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I mean what I mean, but if you know what I mean, how can you mean what I mean - y'know what I mean?