I have a Sorin Markov. By far my favorite planeswalker. The one planse walker who can easily kill other planeswalkers. Also a really fun way to take care of those pesky lifegain decks. "Oh you have a billion life? Now you have ten..."
I also have soooo many Vampires. They were what I started with and have a special place in my heart... Agro Vampires are so fun~ I had a really cool red/black Vampire deck durring the Innistrad block a couple years ago.