This is going to be a hard pick, as I generally like most of the Pokemon. Here goes nothing.
Bug: Swadloon. It always looked a little like a dumpling wrapped in a leaf blanket, and its eys just seemed weird. Whatever happened to that cute Sewaddle?
Dark: Vullaby. I love Mandibuzz, but Vullaby just doesn't cut it for me. Being a chick Pokemon, you'd expect it to be at least somewhat cute.
Dragon: Shelgon. I don't really like its design, with it just being a giant white ball with eyes and stubby feet.
Electric: Pachirisu. With terrible stats all across the board and an unoriginal design, the only thing good about it is it's cute physique.
Fighting: Throh and Sawk. I am not a fan of human-like Pokemon, and these two are no exception. Thier designs and concept seem so underdeveloped and lazy on Nintendo's part.
Fire: Pignite. I like the rest of Tepig's evolutionary family, but it's just Pignite that urks me. He's a chubby, derpy pig. And he's a fire/fighting type. We have plently of fire/fighting starters.
Flying: Ledyba. It's so puny and ridiculous looking, with terrible stats to boot. I like ladybugs and this is a disgrace to them.
Ghost: Misdreavus. I don't know why, but I never liked its design much. Trainers utilize this Pokemon to be such a pain (Confuse Ray, Pain Split, etc.). All around not a fun Pokemon to fight.
Grass: Sunkern. Isn't it obvious?
Ground: Dugtrio. How original. The only good stat it even has is speed, which doesn't help with its pitiful defenses and below-average attack.
Ice: Jynx. I really don't like human-like Pokemon at all. They just seem out of place in a game centered around creatures based off animals.
Normal: Happiny. Its design is so bland. The only saving grace it has is the amount of experience points it gives.
Poison: Muk. There was nothing else the creators could do when thinking of the transition between Grimer and its evolution?
Psychic: Unown. It's so painfully bad that I don't even want to write an explanation.
Rock: Bonsly. I was never fond of the baby Pokemon, and this thing just creeps me out. Like all baby Pokemon, terrible stats and moves. Not much else to say.
Steel: Klang and Klinklang. I like originality in my Pokemon. Klink was cool when it was released, but its evolutions suffer the same fate as Muk.
Water: Luvdisc. No one likes Luvdisc. Terrible stats, poor design, and it didn't even evolve into Alomomola. It does have a decent concept, though.