When it comes to my favorite Pokemon, it's pretty much a tie between Cacturne and Gourgeist.
When I first saw Cacnea, I thought it somewhat resembled a jack-o'-lantern creature, which was only strengthened by seeing that its shiny sprite was orange. That the, in my mind, jack-o'-lantern monster evolved into what resembled a scarecrow was a great design decision as far as I was concerned. As you can probably tell from my name, I'm a fan of the horror genre, and I am also very fond of the season of Autumn. For years, because of my own little eccentric reasoning, I figured Cacturne would be the closest thing to a jack-o'-lantern or anything to do with Halloween that we would see in the Pokemon franchise, so I cherished it.
Imagine my surprise when I found out that we would actually get a pumpkin Pokemon in Pumpkaboo in X & Y. I would have loved that enough on its own, but I am really glad that Pumpkaboo actually evolves into Gourgeist and that Gourgeist literally has a jack-o'-lantern as its base. I was originally a bit turned off and disappointed by the top portion but I came to like it in time.
Neither of them are perfect. They're both horrendously slow (the competitively viable Super Size Gourgeist, anyway), which is especially bad for Cacturne since it also has terrible survivability. Meanwhile, Gourgeist lacks access to Shadow Claw, but I'm a bit less bothered by that now that Z-Moves are a thing. But who cares, nothing in life is perfect, and their existence brings joy to my life. Plus, hey, Gourgeist even has a move called Trick or Treat. That's great.
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As far as least favorites are concerned, I don't really strongly dislike any single Pokemon. Some of them might be essentially useless competitively, but that's fine by me.
That said, some Pokemon do bug me through no real fault of their own.
I do not understand why Lucario is so popular in the Pokemon community and it sort of feels like it's only popular because of its popularity (as in, some people only say they like it because of its reputation for being cool.) Its design isn't bad, but it's also nothing particularly outstanding. Its typing is neat, but there are many other Fighting types I tend to favor using instead of it.
I'm also kind of annoyed by Pikachu, but not because it is the mascot. I'm more annoyed with it because it constantly overshadows Raichu and even has its own hold item to improve its battling to the point that you might as well not even bother evolving Pikachu into Raichu in the first place. I suppose this gripe is more with Game Freak and the people who worked on that episode of the anime with Lt. Surge. Considering the trend of electric rodents in each new region, Raichu probably would have been better off as its own standalone creature entirely separate from Pikachu, but what's done is done. At least it got some attention in Sun & Moon.