Red+Blue+Yellow: Chose the mascot on the front.
Silver: Chikorita, b/c it was our favorite
Crystal: Cyndaquil b/c why not? It was a fire type, so that's what we went with
Sapphire: Mudkip, b/c we liked it, and it was blue, like sapphire
Ruby: Torchic, b/c we liked it second best, and it was red, like a ruby
Emerald: Treeko, b/c that's what was left and he was green
Firered and Leafgreen: Mascots
Diamond: Chimchar, liked him the best.
Platinum: Turtwig b/c I could trade Piplup over from Diamond.
SoulSilver: Chikorita b/c nostalgia.
White: Oshawott, b/c he actually looked cute.
White 2: Snivy, b/c I hate Tepig line
X: Chespin b/c...I don't know. I was close to picking Fennekin, but I thought I would give Chespin a try.