I love questions like these :D
Gen I: Victreebel. My little Ronald was my prize Pokemon! And he was the one who beat Misty hah.
Gen II: Heracross and Wobbuffet. I loved that blob in the anime and I miss it.
Gen III: So hard! I loved Gen III! Gardevoir.
Gen IV: Garchomp
Gen V: Darmanitan!