I have three favorites and a horrible inability to make decisions. My favorites are Cacturne, Gourgeist, and Escavalier.
- Before X & Y released, Cacnea felt like the closest thing to a jack-o'-lantern in the series, especially since its shiny is orange, so I latched onto it. Then Cacnea evolves into Cacturne, whose design evokes the idea of a scarecrow, and I thought that was really neat. Cacturne has really great attacking stats and some good moves, but it also unfortunately has awful survivability due to its low Speed and non-existent HP, Defense, and Special Defense. Regardless, I still think it's cool.
- Then X & Y came along and brought us Pumpkaboo and Gourgeist and I obviously loved them. Pumpkaboo's got that cute cat face and the little wings, then it evolves into Gourgeist with that full-on jack-o'-lantern as its body, absolutely delightful. I also love how they have four different sizes in Small, Average, Large, and Super, and they all have their own stat spreads. The only thing keeping Gourgeist from beating out Cacturne is that Gourgeist's movepool is awful; it has barely any good physical attacking moves to take advantage of its good Attack stat, it has a ton of options for special attacking moves with a terrible Special Attack stat of 58, and thus you're pretty limited in terms of options for using it. Basically just useful for Support, but even then it's pretty limited and predictable. I've always been disappointed that it doesn't have access to Shadow Claw since that would be an actually good physical Ghost-type attack, and if it actually got that then it'd definitely beat out Cacturne. Alas.
- As for Escavalier, I'm fond of the aesthetic of knights and armor so I think Escavalier's armor and jousting lances are really cool. Awfully slow, but that can be remedied or worked around in some ways, it has nice stats otherwise and its Bug/Steel typing means it's only weak to Fire-types. Escavalier's definitely my favorite to actually use out of these three.
If I had to choose just one of them then, having written it out here, the obvious choice is probably Escavalier. I prefer Gourgiest aesthetically since it's the Halloween Pokémon, but I just don't enjoy using it as much.
Honorable mentions for Rowlet (precious borb), Blacephalon (I like clowns and it's really cool type-wise and conceptually with the bomb head), Palossand (cool typing and I love the concept and design), and Malamar (overall just neat).