What is the beast game for GBA?

Chrono Trigger or Pokemon Platinum.

Professor Layton and the Curious Villiage is cool if you like getting super pissed over something very simple.

Pokemon Platinum is for the Nintendo DS only I believe.

For the GBA, any pokemon game will keep you busy for a long time. My favorite was Pokemon Ruby Version. I did like Emerald as well.

Here is a top 100 list of GBA games.

I totally missed the GBA part of your question. My apologies.

Golden Sun parts one and two or Warioware.

Super Mario World! (the version for GBA) I’m ADDICTED to that game!
Probably one of my favorite games of all time!

Castlevania Circle of the moon.

Yoshi’s Island! <3

Edited to add: also Fire Emblem :)

I’ve read that WarioWare: Twisted! (the only one I haven’t played) is the best in the series, so you can check that out instead of the original one.

If you missed them the first time around, you should play Yoshi’s Island and Legend of Zelda: A Link To The Past (with a bonus game, 4 swords, on it!).

Drill Dozer and Pokemon are pretty good, too. :)