Gen 1 - I don't know. Maybe Metapod?
Gen 2 - Magcargo. Seriously, it looks like someone took a pile of lava and slapped a round rock and a chicken's head on it. Gross.
Gen 3 - Delcatty. Its design is too freaky for me.
Gen 4 - Bibarel. Purugly may have ugly in its name, but Bibarel... ugh!
Gen 5 - Watchog, Simisear, Simipour, and Garbodor. Can't choose between either, because they're all so damn ugly!
Gen 6 - Chespin. While a mammal grass starter was a nice change, I don't really like Chespin's design. I really don't like seeing it with its mouth open!