Honestly, I find Torterra from D/P to be the best - not just because of my emotional connection with him (it literally took me a full year to finish the elite four - anybody who played D/P knows why), but because all around he was a pretty ridiculous pokemon. I loved the tank style, and though speed didn't always matter to me, there was always EV training to get that stat to be equal with the others. Overall, though he was my starter, once I finished the game he became my cherished "last resort" pokemon, and I loved him to bits.
Plus, you have to admit Turtwig is super kawaii, especially for the amazing pokemon he turns into later: