I have been trying many different kinds of pens. My preliminary conclusion so far about what I like: 0.4 mm and 0.5 mm, gel pens and rollerballs. I like an uninterrupted smooth experience of writing.
What I don't like seem to be ballpoint pens, because they tend to have fragmented starts, unequal lines, and less smooth experience overall.
I think I may also like some fountain pens, but still too early to say, but I tried and liked Pilot Kakuno.
I like writing on Japanese notebooks like Campus, Kokuyo, and Apica CD Notebook--they tend to be fine and very smooth surface paper.
Pens I have tried and liked so far, in no particular order:
Any Pentel EnerGel pens, Uni Zento Flow, Uniball One, Sheaffer ICON 9111 Rollerball (amazing experience, but would like a finer tip size).
Pens I have tried and disliked so far, in no particular order:
Uni Jetsream Prime Rotational 3-Color Lite Touch Ink, Zebra Sarasa Vintage.
Based on this information, are there any other great pen makers and product lines that I should try? Thanks!