Same one I've always used ever since my comic book detour. The Staedtler Mars Graphic 3000. It's the closest thing to a real brush as far as "feel" goes. Most brush pens are simply too stiff to really simulate an actual brush. Unfortunately, the ink is not the permanent kind. But since I scan everything in anyways - it doesn't make that much difference.
I first discovered the pen from John Byrne's letter page in FF when I had an actual subscription!