I'm a crazy dude that prefers 3D World. Haven't played 3D Land. I like the rest except maybe Odyssey, which is mostly boring outside of its main adventure.

I think 64 is the best Mario 3D. I don’t know why the majority prefers galaxy. Idunno maybe it’s a generational thing.

Mario 64, Sunshine, and Odyssey, aren’t a subseries.

Agreed.  Odyssey and 64 are great.  Sunshine is the worst 3D Mario game.  

Basically...

Mario 64/Odyssey/Sunshine to a lesser extent- You are given a vague hint or direction, and explore a world to find a star/shine/moon. Often you have to complete some objective to trigger the star (i.e. stomp on the wooden stake attached to chomp's chain, track down five Boos, beat a Koopa in a race, etc.)

Mario 3D Land/World- You go to the end of the level. In most levels, there is no question of where exactly you're supposed to go, and the only challenge is getting there. You do not have to do anything beyond simply getting to the goal and unless you're a completionism, there's no reason to hunt around for anything hidden. 

Mario Galaxy 1/2- Sort of a hybrid between the two. For some stars you follow a set path and your only objective is to make it to the end. For other stars, you have free reign of the level, and have to complete some objective (finding parts of a launch star, purple coins, destroying a giant robot, etc.) to make the star appear. Most levels are a sort of hybrid. You traverse a series of small self contained areas (usually planetoids) in a clear straightforward path. However, typically you have to accomplish some small challenge on each planetoid (beat all the enemies, find a key, find pieces of a launch star, etc.) to get to the next area.

All three styles work great. I prefer Galaxy the best. It finds a way to give you freedom while preventing you from having to wander around poking at things to find what you need. Focusing on smaller levels allows them to throw in tons of new ideas and then move on before they've worn out their welcome. The game's structure lets them change mechanics or setting on the fly to constantly surprise the player. It's really amazing how many different ways they find to make running and jumping feel fresh and exciting.

Personally Mario Galaxy 2>>>Galaxy 1>>64>>>3D World>Odyssey>>3D Land>>Sunshine. Of course they're all, except probably Sunshine, 9/10 or above games. 

3D Land Benefits from not having to accommodate 4 players, so it feels a lot tighter when you're playing single player. On the other hand though, 3D World has a lot more creativity in its level design. 3D Land is a series of fun but pretty straightforward levels. Whereas 3D World has a lot of "gimmick" levels. I personally prefer the latter, but 3D World is still a really solid platformer. It kind of feels like Mario Bros 3 extrapolated into a 3D game, and that's not a bad thing.

Exploratory for sure. The combination of exploration and platforming in these games is perfection. Plus, the move sets in 64, Sunshine, and Odyssey are, in my opinion, superior to any other game in the series.

Yeah, this distinction is pretty arbitrary. This is a weird thread.

Galaxy for me.

Not really. There's a huge difference in them from a game design perspective. They're far more different than most sequels are.