Best (Pre Nintendo Switch) Handheld Game of All Time?

[Signalstar]

Now that the Nintendo Switch is here I guess all of its games can be considered console and handheld titles. The distinction used to be clearer however and there was a perception that purely handheld games were inferior to console games. That being said what handheld game do you think is the best of all time? Obviously Switch games don't count for the purpose of this thread.

 

I will say Pokemon Red/Blue.