Gen I:
Farfetch'd - It has a boring design, doesn't evolve, and is useless in battle.
Mr. Mime - Hideously creepy... I hate that thing. =/
Gen II:
Ledyba - It looks really weird, but I love Ledian. :x
Girafarig - I never payed any attention to it, and I never found it interesting...
Granbull - How can something that was once so cute become so ugly? And you can imagine how shocked I was when I realized that it has a 25% male/75% female gender ratio. X_x
Stantler - Its face looks like a butt, and its Hypnosis + Stomp combination is really aggravating. Dx
Gen III:
Ludicolo - I used to like it... but as soon as I saw it in action on PBR... that was it for me. ._.
Nosepass - Its cry is ugly, its face is ugly, and hell, it's a NOSE compass. That's just... ew. =/
Beldum - I hate how it can only use Take Down until it evolves, and its design just... baffles me for some reason...
Gen IV:
Turtwig/Grotle/Torterra - THE worst starter Pokemon I've ever seen. Heck, we've already got a turtle starter anyways. And the double weakness to Ice that it gains from becoming a Torterra is NOT cool.
Starly/Staravia/Staraptor - Probably the worst regional bird yet. I just hate them all with a passion. I just don't know why, though. '_'
Kricketot/Kricketot - Worst regional bug. It's boring, doesn't learn many good moves, and I just simply don't like it.
Bastiodon - Ugh, I try to like it, since I love Shieldon, but dang this thing is just TOO ugly to like.
Buizel - I don't see why it's loved more than Floatzel. Floatzel is WAY better in Buizel IMO.
Ambipom - They killed Aipom. KILLED IT. Even though I've used Ambipom twice, I just can't seem to get over how ugly it is compared to Aipom. See? THAT'S why I hate it when Pokemon I like get bad evolutions. I become tempted to evolve it anyways. T^T
Lopunny - It's as if it's trying to be the new Gardevior... but it's not working. It's just awkward looking...
Mismagius - Uh, Misdreavus did NOT need an evolution, GF. Mismagius is creepy as hell, and what's up with its neck?! Dx
Bronzor/Bronzong - 'Nuff said.
Spiritomb - It's just trying to snatch Sableye's glory. u_u
Lucario - I will never understand why this thing is so overrated. It has one of the most ridiculous fan bases I've ever seen. No offense. =/ It also annoys me how, for no reason whatsoever, it has the ability to talk in the movie, but every other Lucario shown can't. T_T
Skorupi/Drapion - Their names and designs just don't fancy me at all. It was also a member of one of the worst teams I've ever made. :x
Abomasnow - It's just SO awkward looking that I hate it. xD
Magnezone - Magnezone... My god, GF, you really ruined the Magnemite line with this monster. It's like... a Magneton that melted into a big blob of Magnemites.
Rhyperior - I always hated this thing. Why was it even created. Rhydon was fine the way it was.
Tangrowth - I can't believe I actually used this thing. It's hideous, and makes Tangela look even worse than it already did. Heck, it's just a larger Tangela with arms.
Magmortar - Jeez, I hate Magmortar. It destroyed Magmar for me. Now every time I use Magmar, I'll be tempted to evolve it into Magmortar for the extra strength. Even though I despise it. :P
Probobass - And I thought Nosepass couldn't get any uglier. T_T But on the plus side, it does have a pretty interesting cry. :>
Dialga/Palkia - The absolutely worst legendary pair ever. Their designs are way too complex for me to like them. The whole time and space legends idea could've been executed better. =/
Giratina - BARF. Like Dialga and Palkia, it's too complex, and I don't like its background story. At all.
Heatran - UGH. I can't explain how much I hate this legendary. WHY is it even a legendary to begin with? We already have a volcano legend, Heatran. It's called Entei.
Regigigas - The regis were fine as a trio. A leader was not needed. Besides, why is it even a Normal type? I could've sworn it was a Grass or Ground type. :(
Cresselia - Firstly, can someone please explain to me how it's Darkrai's opposite? And it's so easily overlooked that I just don't like it.
Arceus - Wow. Just wow. I can NOT believe that GF actually created a GOD Pokemon. Isn't that a bit religious? It doesn't make sense to me how a Pokemon is said to create every other Pokemon. It also sounded like such a queer in the movie. I could NOT take it seriously. x)
Gen V:
Emboar - Well first of all, did we really need another Fire/Fighting Fire type starter? Yes, I know Pignite is also Fire/Fighting, but at least its cute design makes up for it, but Emboar. Oh, Emboar. Why? Just why? Its arms are WAY too large and long, its head is way too small, and the little "flaming beard" it has makes it look really lame, tbh. Regardless of how much I hate it, I already know once I get Black, I'll end up evolving my Pignite into an Emboar due to the lack of power. =/
Oshawott/Samurott - Oshawott's fan base is just so two-faced. When it was first revealed, nearly everyone hated it, but as soon as it's shown in the anime, pretty much everyone falls in love with it and starts calling it "SOO KAWAII" and stupid crap like that. Then its evolutions were shown. I never had a problem with Dewott (except its color scheme), but Samurott was, and will always be, a HUGE disappointment to me. Lots of people think it's all cool and stuff, but I just think it's hideous, awkward to look at/ and definitely needed the added Fighting type more than Emboar. u_u
Simipour - Bleh, I like Panpour, but it becomes way to creepy looking as a Simipour. I also hate its pose. =/
Audino - Another Pokemon that just tries to be cute but fails at it. It's seriously one of the creepiest Pokemon I've ever seen. D:,
Sigilyph - Looks like a long lost evolution of Unown... yet it doesn't evolve from it, and doesn't evolve at all. I also just don't like how it looks. Heh, kinda looks like a pinata. xB
Archeops - I dislike its snake-like head. If it had some more hair/fur(?) on its head, then I'd like it more.
Vanilluxe - I don't need to explain. >.>
Klang/Klinklang - What the hell happens here? One little ring is added to the bottom of Klang, and it becomes a completely new species? That's just sad.
Cobalion - Eeee... this Pokemon just scares the crap outta me for some reason. I hate its sprite's movement as well. =/
Tornadus/Thundurus/Landorus - Wait, those are seriously their English names? ...Weird. ANYWAYS, it's pretty much obvious to why I dislike these three. To start things off, there's already TWO other legendary trios, and then there's the extremely lazy design of these three. It's just the same Pokemon with a different color scheme, beard, and tail (those ARE their tails, right? O_o).
Kyurem - I might stop hating it so much if it gets a second form. I just hope that doesn't become a theme with the third legend in future Pokemon games. >.>
Heheh, sorry folks, but there's just too much to hate to list only one. :P