My absolute favorites, based mostly on aesthetics and typings since they have such terribly low Speed stats, are Cacturne and Gourgeist (and their pre-evolutions Cacnea and Pumpkaboo). I've gone over why that is before in other threads, but basically, Halloween is my favorite holiday and I love that Gourgeist exists so we can have an actual jack-o'-lantern Pok?mon. Meanwhile, while Cacnea and Cacturne are obviously cacti instead of pumpkins with their normal green coloration, their shiny coloration is orange and before X & Y they were the closest thing we had to jack-o'-lanterns with the facial design and their shinies sort of make me think of a jack-o'lantern monsters. Plus Cacturne's scarecrow design is really nice. If I had to pick one over the other, it'd probably be Cacturne strictly for moveset, but Gourgeist is admittedly more useful with its bulk. It just bugs me that Gourgeist can't learn Shadow Claw.
I don't really strongly dislike any Pok?mon, but after the previous two, there are definitely some that I prefer over the rest of the roster. In alphabetical order:
Aegislash
Aerodactyl
Araquanid
Articuno
Beheeyem
Blacephalon
Blastoise
Cofagrigus
Escavalier
Exeggutor (I actually prefer the original over Alolan, but both are good)
Fearow
Hawlucha
Kabutops
Kyurem (I like normal Kyurem better than Black or White Kyurem)
Lycanroc Midnight Form
Machamp
Malamar
Mew
Moltres
Muk
Murkrow (and Honchkrow, but I prefer the witch hat and broom tail interpretation of Murkrow over the gangster one)
Omastar
Palossand
Parasect
Pheromosa
Porygon2
Poryon-Z
Primeape
Regigigas and the other Regis (I love Regigigas' design even though it sucks to use)
Rowlet (and its evolutions, but Rowlet is adorable)
Serperior
Trevenant
Vespiquen
Vileplume
There are some more that I can list that I like just as much as the ones I just listed, but the list is long enough already.
As for my favorite game in the series, tough call between HeartGold/SoulSilver, X & Y, or Omega Ruby/Alpha Sapphire. HGSS lack modern conveniences, but I love traveling Johto, having your Pok?mon follow your trainer, and having a whole second region to explore to earn 16 badges. X & Y, while unfortunately very easy, introduced trainer customization and Super Training, which is my preferred way to EV train my Pok?mon. ORAS retains the Super Training and has more challenging gameplay than X & Y, but lacks the trainer customization. Special shoutout to Black/White/Black 2/White 2 because I love Unova.
I'd probably have to give it to Omega Ruby/Alpha Sapphire, particularly Alpha Sapphire because I like Archie way more than Maxie. It's unfortunate that it lacks trainer customization, but I mean, it makes up for it in every other regard.
