Galvantula is extremely good. Like, really good. When I played Pokemon White for the first time, I used it and it did an incredible job.
Absol is kind of similar in that I used it to wreck SoulSilver. That Attack stat is insane. It also looks pretty cool to me.
I chose Latios because it's one of few Legendary Pokemon I actually like...and I don't really like most of the regular dragons barring Altaria and Latias, which couldn't make it to the list because of this guy.
Pikachu, well. It's actually a hard decision for me since I like Dedenne and Emolga a lot. I went with the Pika because I associate with the Pika.
I haven't actually used Sylveon yet, but I really want to. It reminds me of Glaceon, which I am a fan of, in terms of stats and it has a nice appearance.
I don't like fighting types, if anyone was wondering why I don't have anything for three regions.
Pancham, however, will reck u m8, and he has this Hamtaro face that just really hits it off with me. Not literally though because that would hurt.
I want to see Flareon get a better movepool someday. I actually like its stats and enjoyed using it in a few playthroughs. Arcanine and Ninetales get special mentions, as I like them more than the other fire types on this list, but Flareon overshadowed them.
(Forget Fennekin's last form, btw.)
Tropius has one of my favorite designs (and typing that interests me) in all of Pokemon, so of course I'm going to select it as my favorite flying type!
Galvantula wasn't the only Pokemon that I enjoyed using in my Pokemon White run. Jellicent did a wonderful job as well. Very fun to use and has a more interesting designs (and typing) than other ghost types in my opinion.
Everyone ever hates Meganium apparently, but I don't. I think it has great stats, I like its design, and it learns Earthquake, which is more than enough to deal with in-game stuff (and I only play in-game Pokemon).
Nidoking is one of my favorite Pokemon to use in any Pokemon game. So much so that I had to ban it from any future runs I do. I love its stats, diverse movepool, and typing. The design is sort of neat too.
Walrein is ALSO one of my favorite Pokemon to use. I prefer it over Lapras, which is pretty much the same thing due to its design, mostly. 3rd gen in general has my favorite Pokemon designs.
Zangoose is my favorite Pokemon, but there was no way I couldn't put Cinccino next to it for favorite Normal type. I like Zangoose's speed and diverse movepool. I like its design. I like its signature attack. It just does such a good job destroying stuff for me in in-game playthroughs.
Cinccino's Skill Link ability literally saved A LOT of time for me when dealing with Pokemon with Sturdy. I thought Sturdy was the most annoying thing in Pokemon (besides Stealth Rock in particular and all other traps that literally last forever) until Cinccino showed me a way out of it. It was also part of that Pokemon White team. So powerful and fast for a little guy.
Despite how annoying Zubat is to a lot of people, I don't think anyone could ever say Crobat is a bad Pokemon. It has a great movepool and really good stats that make my playthroughs with it quite enjoyable. Heat Wave is silly for it to have, but oh so useful.
Grumpig's my favorite Psychic type because it has a good moveset, like the other Pokemon in this list and is also very rare to find in in-game Pokemon. It doesn't make a single appearance in the games its from, so you wouldn't know it existed unless you used Spoink, which is only found in one of the less memorable areas in the game: Jagged Pass. I also like Chimecho for also being oscure, but again. It was overshadowed.
Armaldo has one of my favorite designs in the game as well one of my favorite moves in the game: Crush Claw (note that that's Zangoose's signature move). Can't say much else. It's really the design that gets me.
Steelix, as well. Design choice. However, Steelix and I go way back. I've never actually used one before, but when I played Gold, I got the Onix you can get as a trade, but didn't know it had to be holding an item during it to evolve. I used that Onix from the beginning of the game and basically solo'd most battles with it. I never gave up hope that it might somehow evolve until it hit level 100 before I battled Red. Someday I'd love to use Steelix in a team for nostalgia reasons.
Oh man. Lanturn is such a good Pokemon. I love the whole Water/Electricity typing it has, as well as Volt Absorb. That meant it was only weak to Ground, which it could easily take care of, and Grass, which it can use Ice Beam. Very tanky with moves that compliment it well.
Annnnnnnd Yveltal has my favorite design out of ANY Pokemon, so it gets the legendary Pokemon spot.