BUG: Mothim (Illumise)
I love Mothim's Shiny colors, plus it's an interesting but often forgotten Pokémon. As for Illumise, I've just never cared for its humanoid design (same with Volbeat, but Volbeat's XYORAS model looks decent).
DARK: Sneasel (Malamar)
I've always thought Sneasel was pretty cool, especially since my cousin flaunted around his Neo Genesis Sneasel back in elementary school. Malamar is just ugly and awkward.
DRAGON: Dragonair (Garchomp)
If I had to choose the most beautiful Pokémon, it wouldn't be Milotic. Dragonair is just gorgeous. I've never liked Garchomp's design, but you see so many people using it. =_=
ELECTRIC: Raikou (Eelectrik)
Raikou is just cool, okay? Eelectrik is awkward.
FAIRY: Togetic (Aromatisse)
Togetic brings back nostalgic memories of playing Pokémon Silver (my first game of the series). Aromatisse is kind of scary-looking and with its leg sticking out like that, it looks like it's trying to seduce you which is just all kinds of wrong.
FIGHTING: Tyrogue (Pignite)
Tyrogue is such a cool little guy and his Shiny is amongst my favorites visually. I think it's pretty cool how tricky it was to get him in Gold and Silver compared to the other Baby Pokémon. Definitely my favorite Baby Pokémon. As for Pignite, I think it's another awkward design.
FIRE: Ho-Oh (Magmortar)
I had considered Ho-Oh my favorite Pokémon for a long time. Not only is it a phoenix (my favorite mythological creature), but I also used it to take down my cousin (who I looked up to as a "god of games" back then). It's not my favorite anymore, but it's still pretty close to the top of my list. I really like Magmar as well, but Magmortar ruined it. So much wasted potential. T_T
FLYING: Ho-Oh (Pelipper/Vullaby)
See FIRE for Ho-Oh. Just looking at Pelipper makes me feel annoyed... not sure if it's the eyes or recalling the massive amounts of them in RSE. Vullaby is an ugly, shriveled-up thing.
GHOST: Misdreavus/Froslass (Dusclops)
Misdreavus is gosh-darned adorable, but I grew attached to Froslass in Pearl, so it's hard to choose between them. Dusclops, imo, has a really dull design and is one of my least-favorite Hoenn-mon.
GRASS: Nuzleaf/Bayleef (Cacturne)
Nuzleaf looks so cool and proud, though I think the female should've had a gender difference other than a shorter leaf (if you know what I mean). Bayleef's mostly for nostalgia (Chikorita was my very first starter).
GROUND: Sandslash (Seismitoad)
Sandslash is just cool. Seismitoad is one of the most-disturbing Pokémon designs ever. Those jiggling things! D:
ICE: Froslass (Beartic)
See GHOST for Froslass. Beartic has a very odd-shaped body. I really dislike the shape of its head. A lot of wasted potential for a polar bear Pokémon, there.
NORMAL: Dunsparce (Lickilicky)
Dunsparce (specifically Shiny Dunsparce) is my favorite Pokémon of all. It's just so weird and squishy-looking. I got attached to it as a kid when I spent forever trying to evolve it into Gligar in vain. It wasn't until many years later that it became my favorite after finding a Shiny one in Pearl on Christmas. Lickitung is another one of my top-favorite Pokémon, but like Magmar, the evolution is terrible. The only good thing Lickilicky did for the line was enable Lickitung to use Eviolite.
POISON: Nidoran M (Garbodor)
Nidoran M is adorable, plus I found a Shiny one in FireRed. I'm aware that Pokémon means "Pocket Monsters," and Garbodor is one of the finest examples of a monster, but I prefer Pokémon that aren't based on exploded bags of trash.
PSYCHIC: Mew (Malamar)
Mew is so cute and mysterious. Malamar is... well, see DARK.
ROCK: Corsola (Binacle/Barbaracle)
Corsola is cute. Binacle and Barbaracle aren't.
STEEL: Jirachi (Bronzor)
I don't really like the Steel-type, but Jirachi is one of the few exceptions because it actually looks cute. Bronzor, who I considered my least-favorite Pokémon for a while, just looks stupid. It looks like it has a clown nose. :|
WATER: Wartortle (Seismitoad)
Strictly design-wise, Wartortle would have to be my favorite Pokémon. Those fuzzy ears. That fluffy tail. That blush. Those little fangs. That tough look in its eyes... When I see it, I squeal like a fangirl a little on the inside. It's just one of those designs that makes me smile. Seismitoad is one that makes me want to puke. :)