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So far, it's roughly even. All-time favourite is the Chikorita line, choose Squirtle for Kanto, Torchic for Hoenn, and Turtwig for Sinnoh, I've used all three for Unova and like them all (used Snivy for my first White run, Oshawott for White 2, and Tepig for my second White run), and I'm leaning towards Fennekin for gen 6 (although I'm waiting to see final evos/types - if Chespin ends up Grass/Dark, I'm going to pick that).
So, not including gen 6 and including all three Unova starters, that's three Grass types, two Water types, and two Fire types.