Well, since people seem to be very open about their favorite Pokemon, I figured to look inside the heads of these people like the creepy geezer I am.
Just say what your favorite Pokemon is and describe why is it your favorite in great detail. I figured this wouldn't fit in the general favorite thread because it's more about reasoning.
Well, I'll start the party with Magmortar(mostly because I explained why I like Rhyperior in another thread).

Let's start with the basics: Magmortar is a cross-generational evolution, a concept I like very much, especially considering Magmar was really bad.
The design is really weird, but it works. Magmortar is really plump, and seems very jolly. This is good, because angsty, deadly serious Pokemon are a dime-a-dozen. The ridiculous cannons make the design all more weirder and more fun, giving him much personality that a lot of Pokemon lack. It also improves upon Magmar, who couldn't seem to decide what it was based on. Magmortar goes all the way, seemingly representing a personification of fire, fitting the Fire-type gig pretty well.
And I like using the thing, and I actually used Magmar on my first and latest Soul Silver run.