Erm.. Well. I suppose I have tons of favourites, so get ready. Itzza has my permission for trying to ruin all of these. I'm only putting a few for that reason, though.
Arcanine: I like dogs, and for whatever reason, this guy resonates with me. It bolsters some really cool moves, stats, and designs.
Skarmory: It's a metal bird. By the way, I like birds, too. It's a grand wall, and has a moveset that could also make it a sweeper, tank, etc.
Zoroark: It completes the chain of [itself], Lucario, and Mewtwo, which are all cool, it looks generally awesome, and it is also basically the "Plot Twist!" Pokémon, when used in battles.
Flygon: I, for whatever reason, trained a Trapinch when I was a kid, and it ened up being the most glorious thing on my Emerald team. It looks cool, and the idea of a Dragonfly being an actual Dragon, is awesome. It was also my favourite Dragon in Pokémon Ranger. >Metagross
Lapras: After I played Pokémon Mystery Dungeon 2, I accumulated a ton of respect for this thing. Then I used it during a playthrough. Magical. It looks great, plays great, and is constantly given awesome roles throughout Pokémon games.
Glaceon, Jolteon, and Espeon: All tied on favouritism, because they all have their own unique strenghts and weaknesses. Also they look cool, and are all part of the same evolutionary family.
Treeko, Grovyle, and Sceptile: Treeko was the first starter I ever received, and it eventually became the first level 100 I had ever trained. Grovyle was awesome in Time, Darkness, and Sky, and actually has taught me a lot about misleads in writing. It has a special place in my heart. Sceptile is awesome, just in total. I watched the Animé for a bit, and like Tyson's Sceptile a bunch more than Ash's. Because Ash is the worst. I love all of them.
Aggron: This thing deserves to be a Pseudo-Legendary, but falls short in Base Stat Totals, which is too bad. It's got great defence, among other things, and was also my Second level 100. I have a picture of all the reasons you should like it, but there is far too much foul language to look at for even a second. Anyway, it's lovely.
Heracross: It deserves much more respect than it gets, being able to annihilate both Psychic, Rock, Dark, Grass, and Steel types, with STAB alone. It's also exceedingly cool-looking, and has one of my overall favourite cries of every Pokémon. I'd recommend it to a friend.
Slaking: Slaking was another Pokémon I've used, and love. It has stats that surpass even legendaries, and STAB on hyperbeam, which pairs very well with it's only drawback, Truant. I don't wanna get too in-depth on all the ways you can destroy things with this guy, because you can figure it out yourself.
Chandelure: I used this in White, and suddenly everything became trivial, because of how strong it became when Lampent evolved into it. It's also a cool, interesting design, and has an awesome type and moveset.
Rayquaza: Emerald was my first Pokémon game, and this monster was the title Pokémon. I love it so much it's unhealthy. It's cool all around, can beat down both Kyogre and Groudon, and fifth gen allows it to have V-create. The most powerful fire type move, ever. Therefore, it can trounce the Ice types that would even consider making a dent on it, in seconds. Also Extremespeed is a cool move.
I am aware that I'm a nerd.