Normal - Watchog. For me, this thing stands out as the absolute worst of the regional rodents! Not because it's weak, but because it's hideous and annoying! Its pre-evolution, Patrat, is no better.
Fire - Simisear. I loved Pansage when it was revealed, but the others disappointed me. Besides, we've got the Chimchar line, which is much better.
Water - Basculin. This thing is terrible and it's in nearly every body of water in Unova! At least Magikarp and Tentacool can evolve. But Basculin can't, making it suck. Hopefully, the upcoming generation will fix that!
Grass - Chespin. Don't get me wrong. I don't hate Chespin, I just find it overrated compared to Froakie. Also, I couldn't think of anything else besides Chespin.
Electric - Emolga aka AnnoyingStupidFlyingSquirrel. Nuff said.
Ice - Delibird. It's not terrible, it's just meh.
Fighting - Emboar. The fact that it's the third Fire/Fighting starter in a row just rubbed me the wrong way.
Poison - Garbodor. This thing is just pure garbage.
Ground - Stunfisk. This thing is just a pain in the ***! Trying to obtain it in Super Pokemon Rumble was very annoying! Also, Stunfisk is overrated IMO.
Flying - Tranquill. For me, this is the worst Normal/Flying ever! It's not cute, not strong, really annoying, and I don't like its design. I don't like Unfezant either.
Psychic - Unown. This shouldn't have been a Pokemon!
Bug - Combee. Getting a good Combee was a massive pain in the *** back in DPPt. You could only find Combee in Honey Trees, but the chance of finding one was just 20%! Also, the chance of getting a male Combee was 87,5%, and male Combee can't evolve! Nowadays, Combee is much easier to obtain, but it still needs its male evolution!
Rock - Boldore. This thing is just a pain in the *** with Sturdy and Explosion. Graveler may have the same stuff, but at least Sturdy only blocked OHKO moves back when Graveler was abusing Explosion everywhere.
Ghost - Yamask. At first I found this thing adorable. But when I saw that ugly face on its belly, I started to like Yamask less. It's not terrible, though.
Dragon - Druddigon. It's pretty cool, but all other Dragon-type Pokemon are better.
Dark - Mightyena. In the third generation, it was so annoying with Intimidate and Roar!
Steel - Klink. With a noobish design, a shallow movepool, absurd looks, and okay evolutions, Klink just sucks.