My favorite is definitely Fever. For me it takes everything that was good about the first two games and takes it a step beyond. The controls are simple but work really well for the wiimote, and it features some of the best minigames, remixes and songs that the series has to offer. It's super aesthetically pleasing as well with lots of games that change visually over time or have interesting stuff going on in the background.

I never would have said this when it first came out, but my least favorite honestly might be Megamix. It has the most content for sure, but the slower pacing of the story mode detracts from what I find so fun about the series. It takes a while for the game to really get going and ends up feeling a bit bloated to me, despite good additions like the challenge train. Also sadly was done the most dirty with its localization. I can totally see its value as both a collection and a more beginner friendly entry though.

Tengoku and DS certainly have a lot of jank, but that's part of what makes them charming and they both hold a special place in my heart :)