Can't decide between these
Super Mario Land 3 : Wario Land
Link's Awakening
Donkey Kong 94
Three great games of all time, not just for GB. I would probably rank them DK, LA and Wario Land.
Era came through in fine form on this thread. Mole Mania, Pinball: Revenge of the Gator, Metroid II ... all top tier. Played a ton of Kirby's Pinball Land, would probably make my top 5.
Tetris is absolutely classic but without battery backup, it may not be as approachable as the other games for me but it plays second to none.
If anyone still plays their 3DS (I do!), highly recommend picking many of these up on Virtual Console.