For me it's got to be Super Mario Galaxy.

As others have stated, it's one of those games that's sublime to the point of near utter perfection. Everything from the tight controls, to the incredibly creative level design, to the wonderful music, to the beautiful visuals, it's superb in every single aspect.

But tying all these brilliant components into a cohesive whole is this effortless magic that somehow recaptures that childhood feeling of exploring playgrounds and backyards while playing make-belief games of adventure and discovery, and manages to distill it into into video game form. I thought that feeling of childlike wonder had been lost to me forever, but Mario Galaxy brought it back.

A true masterpiece.

I will say mario 64 because it was way better than anything existing when it released !!

I sort of see Galaxy 2 as an extension of Galaxy 1 in a similar way to the Star Road section of Super Mario World - those levels are my favourites of the game. While they are separate from the rest of the game, the rest of the game still exists — Star Road is not satisfying outside of the existence of Mario World. My favourite 3D platforming is from Galaxy 2, but Galaxy 1 is where the magic of the universe and the unparalleled story/atmospheric experience is. Combined, SMG1+2 is the best of both worlds.

So even though I do like the levels of 2 mostly better than 1, the experience is significantly less without the experience of 1. If you can only play one of the two, make it Galaxy 1.

If Nintendo makes a Galaxy 3 I really hope they capture everything in one game.