It won't be easy, since I judge mostly on how the Pokemon design, and some for attacks...
• Bug - Ledyba and Ledian, don't look exciting, kind of boring
• Dark - Houndour and Houndoom, I could add a few more, but those two, they look like rottweiler, I really hate that breed, dissapointed they turned them into Pokemon
• Dragon - Latias and lations, design I don't like, and also not a fan of psychic Pokemon
• Electric - Pikachu, overused in the anime, I learn to not like it.
• Fighting - Meditite, doesn't look like a fighter, more like relaxing yoga. Design isn't good
• Fire - Torchic's evolution line, even if it sounds rude, I laughed when I first saw it, a fire chicken? it has the worst design in Hoenn
• Flying - Farfetch'd, boring design, not helpful in battle,
• Ghost - Shuppet, I find not scary for a ghost type, looks more like a halloween decoration
• Grass - Chikorita's entire evolution, has the worst design for me in Johto, in the Anime, Ash's Chikorita was annoying
• Ground - Dugtrio, its just 3 Digletts together, I really dislike their Area Trap, I'm always forced to battle them. Just as bad as Diglett.
• Ice - As much as I love Unova, the worst looking Pokemon in the Pokemon world is Vanillite's entire evolution, its based on food, I laughed when I first saw its design
• Normal - Ditto, looks more like a blob or slime instead of a Pokemon
• Psychic - Unown, weak battler, weaker design, 26 different designs, and none of them are even good
• Poison - Grimer or Muk, like with Ditto, looks more like slime than a Pokemon
• Rock - Nosepass, don't know what to say about it, other than really bad design
• Steel - Again with me Loving 50% of Unova Pokemon, another of the worst was in Unova, that is Klink's evolution. Don't look like Pokemon, more like they belong in a machine
• Water - maybe Luvdisc, doesn't resemble a fish, more a cut out heart from a paper, and added eyes to it, no fins, no fish-like features.
I didn't expect to take me 2 hours to make my list.