Kanto: Bulbasaur (It was my first Pokemon!)
Johto: Cynadquil (I first chose Chikorita in Gold, then reset once I hit the Sprout Tower.)
Hoenn: Treeko (Sceptile is my favorite Pokemon!)
Sinnoh: None (I picked Turtwig, but I don't love any of them really)
Isshu: None (I haven't played the games yet, but I'm going to pick Tsutaja)
With that, my favorite has to be Treeko, Bulbasaur is second. So, with the exception of Cynadquil, I've chosen the grass type in every region... and I plan to continue that tradition.