Music that deserves to be reused.

Elder Princess Shroob Boss Battle Theme (MLPiT)
Cosmic Cove Galaxy (SMG2)
Space Storm Galaxy (SMG2)
Starship Mario (SMG2)
Clockwork Ruins Galaxy (SMG2)
Digg-a Legg Theme (SMG2)
Toad's Factory (MKW)
Toad Park (MSM)
Western Junction (MSM)
Harmony Hustle (MSM)
Smash Skate 2 (MSM)
 
Cosmic Cove (SMG2)
Puzzle Plank (SMG2)
Slipsand (SMG2)
Mushroom Bridge (MK:DD)
Chain Chomp (MK:DD)
X-Naut Fortress (PM:TTYD)

I will list more in later posts
 
Baby Bowser's theme from Yoshi's Island and Stage 9-8 from Donkey Kong '94.

sweet guitar solos
 
Bob-Omb Battlefield (SM64)
Ghost House (SMW)
Story of Stolen Spirits (Paper Mario's intro)
Princess' Distress (Paper Mario)
Tubba Blubba's theme (Paper Mario)
Jolly Roger Bay (SM64)
Banshee Boardwalk (MK64)
Rawk Hawk's battle theme (TTYD)
Petal Meadows (TTYD)
Francis' battle theme (SPM)
Toad's Turnpike (MK64)
Chai Kingdom (Super Mario Land - World 4)
Skyscraper and Big Donut (MK64)
Ghost Valley (Super Mario Kart)
Rainbow Road (MK64)
 
Snifit said:
Rainbow Road (MK64)
Click to expand...

That technially got reused in the form of Mario Kart 7's Rainbow Road for a good part of the song. But I would love it if both the track made it into a Mario Kart for the Wii U AND a fully orchestrated remix of the music.
 
Actually, all the music for Rainbow Road since MKDD is a remix of this. (Although Mario Kart Wii's is quite different, it is still based off of it.)
 
How did we all forget that?

Super Mario Land Underground
Super Mario Bros. 3 Worlds (Desert, Ocean, Giant, Sky, Snow, Pipe)
Super Mario World Worlds (Donut Plains, Vanilla Dome, Forest of Illusions)
 
Let's not forget Super Mario World's starman. It sounded more complex than Super Mario Bros. 3 and others.