I used to always use a black Pilot vball Extra fine, which is one of those inky pens. You can't draw anything decently with a ball point... they skip.

Nowadays, since I draw much less than I used to, I use whichever pen I've gotten from one conference or another. Just last week my Sapo.pt pen from YAPC Lisbon split apart, spring flew out, had to junk it :(