I used to have only one favorite Pokemon, but then I realized the awesomeness of some of the other ones and now I have four. Wish I could have just one though, it kinda proves how much you love that one and think it's special.
Well, my top favorites are; Scizor, Swampert, Zangoose, Sceptile, Sneasel.
Scizor was my sole favorite Pokemon for a long time. I remember seeing it's sprite somewhere and wondering how the heck you were supposed to get one in Gold (I was only seven back then, so I couldn't have known). I also remember seeing the fourth Pokemon movie, the one with Celebi and Suicune (the name escapes me), and Scizor was pretty awesome in that one. I love(d) the scene where it was battling Ash's Bayleef.
Swampert and Sceptile came relatively at the same time. We lived in the states before Ruby and Sapphire were released, meaning I had access to Nintendo Power. I remember seeing images of the three starters, and I was instantly in love with the Treecko line. So when I finally got Sapphire I was itching to get my hands on one of those awesomesauce geckos. Then I came to Brawly, and for whatever reason I simply couldn't beat him (still have trouble with him D:<). So my friend, being the second graders we were, told me that the only way to defeat him is to start with one of the other two starters. I restarted with Mudkip, and this time I actually managed to beat him. I stuck with Mudkip, and he evolved into Swampert. He was my first and only Pokemon so far to reach level 100. Then I started a new game and he was lost in the depths of cyberspace ♥
Sneasel came relatively at the same time as Scizor. In fact, I think it was the same movie that made me like this little bugger as Scizor. If I remember correctly that hunter guy had both in his team. I didn't think of Sneasel as anything special until recently though. Maybe a year ago or so, I started a MD RP where my two main characters were a Scyther and a Sneasel. I think the point was that I wanted two fast, stealthy Pokemon. Anyway, after this I really warmed up to Sneasels. Weavile is cool as well, and I really think Sneasel deserved an evolution, but I still think the original form is best. [/rant]