Pilot G2. I remember we high school debaters used that all the time as flow pens. It's been my pen of choice ever since.
I've upgraded from blue and black though. Now I like to color code my notes in blue, red, purple, black, and green. Helps with those thermodynamics phase diagram curves you know... =P
Oh the 0.5mm ones rock btw. When you write on a notebook it doesn't come out very thin unless you're writing on single sheets on a smooth desk surface. With the 0.7mms you'll get fat lines unless you plan on using pages and pages of notes. The 0.5mm ones help with equations too as you get fractions and integrals and subscripts and superscripts left and right.