These are some of my personal favorites; I tried to limit the number per game.
SMB2 - Character Select, Title
SMB3 - Ending
SML - Muda Theme, Sky/Marine Pop
SML2 - Athletic Theme
SM64 - Slider, Ending
SMS - Delfino Square, Bianco Hills, Pinna Park, Platforms-a-Plenty
SMG - Melty Molten Galaxy, Buoy Base Galaxy, Ghostly Galaxy
SMG2 - Sky Station I & II, Fluffy Bluff Galaxy, Final boss music
SMK - Choco Islands, Vanilla Lakes, Ending, Tournament Lose
MK64 - Rainbow Road
PM - Dark Days in Flower Fields, Crystal King battle
M&L:SS - Welcome to the Beanbean Kingdom! (outskirts theme), Ending
M&L:PiT - Toad Town, Outside Thwomp Volcano, Hollijolli Village outskirts
G&WG3 (if that counts) - Greenhouse
Mario Party 5 - Bustling Noisily
Mario Party 7 - Credits
Mario Paint - Title, Main Themes 1 & 2, Gnat Attack boss
SMBDX - Credits
NSMB - Athletic, Vs. The Plains
To narrow it down further, I guess, I like the Super Mario Kart, Sunshine, Partners in Time, and Galaxies' soundtracks the best, particularly Super Mario Kart and Galaxy 2. Choco Island might be my all-time favorite, I suppose.