If you enjoy the Metroid Games (as did I! 
) I think you'll be a fan of the Castlevania games as well on the handheld. There's three of 'em: Circle of the Moon, Harmony of Dissonance, and Aria of Sorrow.
Circle of the Moon was my first ever Castlevania game, and it was fantastic. It made me into the Castlevania fan I am today.
Never played Harmony of Dissonance, but I'm always on the lookout for it. I heard it was the weakest of the three GBA games, but still a solid game.
I just found Aria of Sorrow a couple of months ago and bought it right away, and It was money well spent. It's a rare Castlevania in that it's part of a direct line (Castlevania: Dawn of Sorrow on the DS), and it's really awesome. Up there with the best I've played on the GBA.
