Pokemon is, of course, a shoo-in. Overall they probably provide the best gameplay value over all gameboy systems, and they have that strong element of multiplayer to them that you really don't find much in handheld games before current-gen games. As for what I personally enjoyed the most, I love love LOVE the Ace Attorney series - now that is a series I can't get enough of. The localization team was apparently amazing because the games are hilarious in English, and the cases are interesting too. Even though not all of them are super difficult, there were definitely cases I got stuck on for a long time and ones that had a surprise ending for me.