ALttP is widely considered to be the best handheld. It is innovative, with deep gameplay and lots of exploring involved in completign the game. I had the most fun getting all the heart pieces (to this day, this is the only Zelda game that I've completed this task in) because they require you to use your head and really look for them.

MC is a more upbeat Zelda, with the graphics of FS/FSA. It's not as deep, but I found playing it more fun that ALttP. Even though it has 5 dungeons as opposed to LttP's 11, you'll have a great time on it. Plus, there are the trophies to collect and the Kinstones to merge.

You really should get both, because I cannot recommend just one.