Gen 1 for me. Charizard's my favorite Pokemon, Blastoise is a freaking awesome design, and Venusaur is pretty solid.
Though, I have plenty of favorites dotted throughout the rest of the gens as well. Gen 5 could've easily been my favorite as a set if Emboar was Fire/Dark instead of Fire/Fighting, Samurott had a Fighting subtype, and all three were just a tiny bit more useful. Gen 2 and 4 are really solid as well, and 3 has the Mudkip and Treecko line. But still, 1 has my all time favorite Pokemon and two other good starters, so I have to go with it.