Unoriginal I may be, but Charizard is my man.
I had a holographic Charizard card. No other trading card will ever be as legendary. Countless six-year olds were rendered uncontrollably jealous by that card (myself included, until I finally got a hold of one).
I have incredibly fond memories of steamrolling through dozens of Team Rocket grunts with solely my level 100 Charizard way back in Pokemon Red Version.
Lets talk game mechanics. Charizard is a solid sweeper, contrary to popular belief that he isn't as good as other top sweepers, like Alakazam or Jolteon. He makes up for his low Sp. Atk (relative to other sweepers) with his high HP and overall bulkiness and general badassery. Depending on the generation of Pokemon that you play, he's capable of learning a huge variety of moves, making him able to easily counter different types.
Plus he breathes fire.
Pick your favorite Pokemon, explain your choice, and add a picture of him/her/it!