MkWii has the best physics, some of the best tracks in the entire series, a thriving community despite being 10 years old and having official support dropped 5 years ago, and is just overall the best game really.
I don't know if it is just nostalgia, but I have played them all and like the N64 version the best. It is fast-paced, not as cluttered as some of the new ones, and has a good balance between relying on driving skills and the usage of extras.
Part of why I like Double Dash best. It's simpler, just enough more complicated than N64 for me. The ability to let a second player take the "gunner" role lets me play it with my kids who are too young to drive well—if they suck at gunner and we're playing on a low enough CC we'll still do OK with me driving :-). I don't like or want online play and certainly not DLC. Pop in the disk, a couple seconds of loading, and go. 8's supposed to be awesome but seems too complicated and tuned-to-suit-hardcore-online-players for my taste.