To try and cover what seems like both ideas that came up in this post (least favorite starter and least liked trait of a favorite starter). I am prefacing this by saying "at the time of the gen's release" because a few mons got a lot of new tricks that changed my opinions a bit (Venusaur and Blaziken, I am looking at you).
Gen 1 - Venusaur was my least favorite; the Charizard line learning Flamethrower so late. Gen 2 - Meganium was my least favorite;Typhlosion's movepool (a bit shallow, but he got Thunder Punch, which was cool). Gen 3 - Swampert was my least favorite; Blaziken was so slow. Gen 4 - Torterra was my least favorite; Infernape was a bit frail. Gen 5 - Emboar... just Emboar; Serperior had little offensive punch (damn fast, though).
Gen 1 - Charizard Gen 2 - Feraligator Gen 3 - None (really) Gen 4 - EDIT! It's actually the monkey. Torterra's just one of my least favorite grass starters, other than Meganium. Gen 5 - the hog Gen 6 - None so far
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