Hmm, I've never really thought of my least favorites from each type before, but here it goes. A majority of these will probably just be based off of design, since there are very few Pokemon I actually hate.
Bug - Metapod. Just a useless Pokemon, unless you evolve a Caterpie into one. (Butterfree's actually my favorite Bug type).
Dark - Sharpedo. I've always hated it's Rough Skin ability, especially when using multi-strike moves.
Dragon - Altaria. Nothing says Dragon better than a cute looking bird with cloudy wings.
Electric - Stunfisk. Honestly can't find much appealing about a flat, land-dwelling electric fish.
Fighting - Monferno/Infernape. Did we honestly need another set of Fire/Fighting starters? Really?
Fire - Pignite/Emboar. Did we honestly need another set of Fire/Fighting starters? Really? (wait, this seems familiar...)
Flying - Gyarados. I actually love Gyarados, but it's always bothered me that it should really be part-Dragon, not Flying.
Ghost - Shedinja. I love to use it, but it's an absolute pain to face if you go in un-prepared.
Grass - Abomasnow. Another Pokemon that I actually like, but it's part-Grass typing just seems so out of place.
Ground - Palpitoad. For some reason, it's stupid wiggling sprite always bothers me. I'm fine with Seismitoad/Tympole, though.
Ice - Cryogonal. Ah, a lifeless snowflake. At least the Vanillish line has the ability to show some emotions.
Normal - Meloetta. I have no problem with Meloetta itself, but there I think a Normal type legendary kind of defeats the purpose of "Normal".
Poison - Tentacool. Out of all the other overly common Pokemon, Tentacool's the only one I hate with a passion.
Psychic - Abra. Don't hate it, but I'm just fearing the day when I find a shiny Abra, and it uses Teleport before I can catch it.
Rock - Roggenrola. Bad name pun, and a design so simplistic that it feels like no effort was put in whatsoever.
Steel - Probopass. An absolutely ridiculous looking Pokemon that doesn't seem to have a reason to be part-Steel type.
Water - Tentacool. Did I mention I hated Tentacool with a passion? Because I do.