Sharpie Pen, but I have used many others and never even had a strong preference for ball point, gel, or felt tip. When I recently ran across a really bad pen, though, I began to understand how important one's choice of writing instrument is.
Of course, I know among the community of people who love journals, there is a strong preference for fountain pens. Obviously, I've never taken any interest in something that specialized.
On a recent trip to New York I came across a really good idea for a pen at Sam Flax. I picked it up as an afterthought-- an impulse purchase at the cash register. I don't remember what the price was, but it was inexpensive enough that it made no impression. The pen was stainless steel, about two inches long, and smaller in radius than a No. 2 pencil. It had a screw-on cap and-- this is the great part-- it was TELESCOPING. Wonderful! A two inch pen that expanded to 5!
The moral of the story for me is that, even if you aren't picky about your pens, there are some minimum requirements for all of us. Personally, I have found that I prefer a pen with a cap (less prone to messy leaks), black ink, and a smooth, even release of ink on the page. I also prefer a fairly thick line so that my writing stands out on the page.
Don't buy the wrong pen for your needs-- a journal is a terrible thing to waste!