Although my avatar may suggest otherwise, definately Sacret of Mana. This was my first ever RPG and one of the best games ever made. I also thought Seiken Denetsu was a great game, though playing it as a ROM on a computer probably took a lot of the feel out of it.

It sucks that they haven't released Legend of Mana here in Australia. I don't have a game boy advance but after what Kawaii Ryukishi posted I'm gonna have to get one for sure.