We all know how it goes. You're struggling hard on a level for hours and finally. You complete the objective and what you hear is the sweet melody of the level clear theme. Post your favorite ones in this threat. I'll start:
This one always made me feel gobsmackingly rich when combined with the sound of the falling coins. It felt like the kid's version of winning a shitload of money at the casino. The theme is simple, but really effective.
After a stessful boss battle (luckily the bosses in this game are few in number and quite easy), there's nothing more satisfying that hearing this. It starts really triumphant, followed by a very calm reprise of the main overworld theme.
Another one from Mario Party, Mario Party Advance this time. A very upbeat remix of the overworld theme of Super Mario World, arguably one of the best. (It sounds much better from your GBA than it does here.)
There are many more, but this is all I can think of right now.