Mario 2D will always be the real classic and the first thing that really comes to mind when you say Mario, at least for me. I loved Galaxy and 64 to death, and even have a special place in my heart for Sunshine. But 2D has taken up even more space. Eh. I've got all of them, though.
Edit: ...I'll never understand people that insist on divvying up their video games like there's some gaming civil war going on in their homes.