Probably Super Mario World. It's the best 2D Mario, which is what you you think of as quintessential Mario gameplay. There's a reason there's so many romhacks for that one specifically. I played all the 2d marios except for the newest (Wonder).

For 3D Marios I don't have an opinion because I've only played Mario 64 and Mario Galaxy 2. Wonky Camera aside, I liked Mario 64 better because the movement feels really fun, even though Galaxy 2 has better levels.

The correct answer is the Mario game I was playing during puberty and nothing will ever change this.

More seriously, Mario 64 was fucking incredible. Just making that leap from 2D to 3D alone was something else. And then they hit it out of the park on game 1. The subsequent Marios were largely solid, but they owe their genes to that original banger.

SMW2 Yoshi's Island is my pick. One of the only platformers that fully brought me in.

Super Mario Land 2: 6 Golden Coins on Gameboy. Haven't played the DX version yet but it looks awesome.

Super Mario World on SNES is a close second.

the only correct answer is New Super Mario Bros DS