Lets see...
Design-wise:
Arcanine - Bland. There are a lot of dog-like Pokémon who look better than it.
Tentacool/Tentacruel - Both have disgusting designs.
Eevee - Its face creeps me out, it is hideous ._. The eeveelutions look cute, though.
Omastar - That mouth >____>
Croconaw - It looks derpy. Feraligatr isn't the prettiest thing, but it looks a bit better at least.
Sunflora - This is probably the Pokémon I loathe the most. Always disliked its retarded design and I always will.
Octillery - Lol, do I really need to explain?
Sceptile - What have they done to you, my poor Grovyle? You are bald. And your tail is a broom ._.
Walrein - Ugh >_>
Wormadam - Worst Bug-type ever?
Pachirisu - I don't think it is cute like people say. Maybe it is because Hikari's Pachirisu used to annoy me in the anime.
Carnivine - And here is the second Pokémon I loathe the most. Ugggly.
Throh - Ugh... Sawk looks fine, though.
Duosion - This thing creeps me out ._.
Vullaby - So damn ugly.
Terrakion - ^ Same.
Pokémon I dislike for other reasons:
Charizard - Overrated, overhyped, etc. Charmeleon is way better than it.
Masquerain - Surkist deserves a better evolution.
I thought I disliked more Pokémon than just nineteen. Oh well...