It's hard to pick. I love 64 and Odyssey, and the open world/semi open world nature of them. Yet I never really got into Sunshine. And even revisiting it on All-stars, it doesn't do much for me and is somewhat irritating actually.
I like the trippy themes and more chill, laid back, and exhilarating nature of the Galaxy games. They're not particularly complex or tough but in a way that's kinda why I like them - when I'm more in the mood to just kick back with a game.
3D World I enjoy for the multiplayer.
I guess overall I'd say the more open-world/exploratory Mario titles are the most enjoyable to me as I tend to gravitate towards the more sandbox-type games. Appreciate the greater freedom of gameplay they offer. It's more in the spirit of the classic Mario games.
"We hold these truths to be self-evident - all men and women created by the, go-you know.. you know the thing!" - Joe Biden
