My favorite Pokémon is basically a tie between Cacturne and Gourgeist and their pre-evolutions Cacnea and Pumpkaboo as well, but I list the evolved forms because I'm more likely to evolve them if I'm going to use them on a team. I like using Cacturne more, but I like Gourgeist more aesthetically for its direct Halloween jack-o-lantern inspiration, so it balances out.
Cacnea, and by extension Cacturne since it's Cacnea's evolution, was my favorite for a few years once I learned of its existence (I dropped the series after Crystal until it was remade as HeartGold and SoulSilver, so I missed the initial run of Ruby/Sapphire/Emerald) and saw that its shiny coloration was orange, which made me instantly think of a jack-o-lantern. This was before Generation 6 and Pumpkaboo and Gourgeist's introductions, so Cacnea and Cacturne were basically my unofficial Halloween representative Pokémon for a while. I really like Cacnea's simple, pleasant design, and I love Cacturne's scarecrow design. The only downside for me is that it has such a terrible Speed stat. I could live with its mediocre HP, Defense, and Special Defense if it could be expected to outspeed things to take advantage of its' great Attack and Special Attack. Unfortunately, it's slow and fragile, which isn't great for a glass cannon. I'm disappointed that it never received a Mega Evolution to patch up its Speed and make it viable. Regardless, I love its Grass/Dark typing and enjoy its movepool.
Pumpkaboo caught my attention once it was released in X & Y and, unsurprisingly, it became an instant favorite. Honestly, it was initially just Pumpkaboo, because I didn't like Gourgeist's design that much at first. In time, though, I grew to appreciate it more and eventually Gourgeist joined Pumpkaboo among my favorites. Gourgeist has almost, but not entirely, the opposite problem that Cacturne has for me; where Cacturne has great offensive stats and bad everything else, Gourgeist has good bulk and a decent Attack stat but doesn't have an actually good Ghost move to use for STAB. Phantom Force is just a Ghost-type version of Fly, a two turn attack, and that sucks in a competitive environment. Shadow Sneak has good priority but is otherwise very weak. It can't even use Shadow Ball, because only its Attack is good offensively, its Special Attack sucks. I'm immensely disappointed that Gourgeist doesn't have access to Shadow Claw, because then it might have actually been my undisputed favorite Pokémon, but it doesn't, and here we are. Nevertheless, I love the way they look and the Grass/Ghost typing they have. Plus they have a move called Trick-or-Treat, that's fantastic.
They're far from perfect, but they're good enough for me.
with dragonair & vaporeon tied for second place
