I've been through gel pens and rollerballs and felt tips and every variation therein and thereof. Today, there's only one pen for me (sort of), and that's the Pilot G2. It's smooth, dries quickly on the pages of my always-around Leuchtturm1917 medium, gridded, softcover book, and retains its blackness for years. Notes written ten years ago look as good and as new as notes written ten days ago.
That said, I've been dabbling in the world of fountain pens and fancy inks (Noodler's is awesome) for some time, and though the writing experience is outstanding, I grew tired of walking around with ink-covered hands, leaving my pen home when I fly (which is often), and waiting too long for ink to dry in my notebook.
So back to the G2 I go, as always.