The mainline 3D Mario games can be kinda separated into 3 categories:

- Exploratory (Mario 64, Sunshine, Odyssey)

- Galaxy 1 & 2

- 3D Land/World

Which of these groups do you like the most, and why?

64/Subshine/Odyssey ftw

It's definitely the Galaxy series for me. Just no-frills platforming amazing-ness.

I voted for 64/Sunshine/Odyssey although Sunshine drags that category down.

Missing the paper ones.
As far as 3D Mario games go, It's definitely the Galaxy series for me anyway. Just no-frills platforming amazing-ness as that other dude just said.

Hence why I said "mainline", referring to the core series rather than the Paper series which is a spinoff.

I prefer the tight level design of 3D World and Galaxy series. The open elements of 64/Sunshine/Odyssey make the experience drag too much for my taste.

What is this explanatory category I've never heard of

And what makes Odyssey any different from Galaxy?

64/Sunshine/Odyssey easily.

Odyssey like 64 and Sunshine is has more open-ended, while Galaxy is separated those by being comprised of more linear courses and its gravity mechanics.