>>51688492Emerald and Firered because I grew up with them. Also because they run at 60 FPS and you can play them on any toaster. Emerald especially is super replayable due to the BF postgame and how well Hoenn is designed in the maingame. I've probably replayed both hundreds of times.
Outside of those two, probably HGSS and Legends. Those two are a lot of fun as well.