... Gen 5.
I had absolutely no interest in Pok?mon for years until a friend almost BEGGED me to play Black and White. I was a bit more interested now, so I picked it up and it was AMAZING. After that, I played the other generations going backwards. (Then played Diamond, then Emerald, etc.)
EDIT: Those were the first games I owned, but a different friend let me try out Diamond and wanted help finding a "blue Gastrodon" but she was checking in the wrong place.
I was really lame as a kid and one time I wasn't listening to my friend so he made me play Diamond because he would threaten to make me play it when I was bad because I hated the idea
Gen 4, Diamant Edition. My cousin was excited about it and showed it to me when I was around 10, I guess. First I was like "meh what's that crap, don't bother me" but then I became addicted until today.
My first Pokemon game was Fire Red, so Gen 3. I miss playing that game. I lost it when we moved, and I don't think it ever made it on the trip with us.
Probably the largest amount of current Pokemon players didn't play those, because of how old the games are. If the franchise didn't pull in new players it would be dead in the water.
Nintendo also made a valiant effort to get all those Gen 1 players back on board with Pokemon Origins accompanying the available Kanto stuff in X/Y... Too bad that Gen 6 is turning out to be pretty much the worst.