In terms of prettiness, it's got to be Dragon Driftway. It's just so rich in colour and effects, and is the only track in the DLC pack where the looks and design wows me more than the racing experience. (not to say that the racing experience is bad, that's awesome too, but I feel like I choose that circuit more for the sight-seeing than the racing) My favourite turn is the left turn where you have the rising (setting?) sun glaring and reflecting on the track, making the glow of the boosts in the centre line more of a challenge to see. I think it's great that Gobblegut from Galaxy 2 features as the dragon.
Mute City is a close second for me. I like how they have the laser MarioKart and F-Zero letters going across the start/finish line, and the details of the city in the background give a real sense of size and height - moreso than the various Rainbow Roads. Of all the tracks in the DLC pack one, Mute City is the only one where I couldn't choose whether the prettiness or the racing experience is better. To me they're equal, and that makes Mute City my personal overall favourite track.
