Lets see here, so many great Mario games to choose from...
3D- Super Mario 64
Super Mario Galaxy
I haven't even played Super Mario Galaxy though, so I am considering Super Mario 64. It was one of the first video games I ever played. I had a lot of fun on nights playing it with my brother's watchful guide over me. The music was so great, and the 3D flare was awesome. I still play it from time to time on my N64.
2D- Super Mario World
Super Mario Bros 3
Ah yes, back to the classics. Don't get me wrong, but I think SMW is personally the best. My favorite part is the wonderfully made music. This game was made on the Snes, so it brought a lot of beautiful color to your screen. A must have.
Super Mario Bros 3 is also an instant classic. After the..uh..SMB2 incident, this game seemed to calm the folks down and give them what they wanted. I like all of the different worlds and themes. Music is a plus. This game was very good and creative considering the limited aspects of the Nes.
RPG- Paper Mario
Paper Mario 2
Here we go, the Paper Mario series. Maybe not as well known as the 2D titles such as SMB3, these games were excellent. Overall, I like the first one better though because it just seemed much more original and classic. Another game I remember playing with my older brother's watchful gaze. We even had a contest to see who could finish PM first. Day and night Paper Mario was running on the screen. The quirky characters, fun battle system, and the loads of sidequests made this an amazing game. When I heard another paper mario was coming out for the Gamecube, I was exstatic. Having beaten the first a couple of times, I was a bit more experienced and ready for a new challenge. I can remember Christmas Day, and playing this on our big screen. Right from the beginning, I noticed graphic improvements, and even more quirky characters. This game is much more retro, and uses a lot more slang. Overall, just as good as the first.