One of the greatest GameBoy Advance games, and RPG's, ever. It starts out with a new villain, Cackletta, stealing Princess Peach's voice. Mario sets off to rescue her voice, and Luigi gets dragged along, as well. If you haven't played it, go find it on eBay now!
Friend: What's your favorite GBA game?
You: Mario & Luigi: Superstar Saga, handsdown.