EpicFail is back! Good to see you, mate.
The Golden Sun series for the GBA just rock. Seriously. Sometimes when I got nothing to do, I grab my old Game Boy SP, and start playing the game. Beaten it like 6 times. It's the second chapter mind me.
I tried 3 cartridges for the first Golden Sun, and all of them were fucked :\ When you turned of the game, all your safe data died, still I managed to beat the game WITHOUT TURNING OFF THE GBA FOR 3 Days lol.
Mario and Luigi Superstar Saga, is one of the greatest GBA games in my opinion. Because it's like a logical sequel to SMRPG (Not a direct one) You play as Luigi and Mario. Fun combat, cool story. I replayed it many times. Give it a try, it's a 2003 title, but it is a revolution in my opinion. Besides I got my Game Boy SP with it :P (Special memories)
And about the classic or modern games.
Honestly I'll go with classics, because they have huge replayabilty and the titles are insanely fun. But I like modern games aswell.