Does it exist? That perfect white pen? I've tried several, and here are the three that I use now:
Ranger's Inkssentials,
Sakura's Gelly Roll, and
Signo's Uniball UM-153. I tested them on papers that I use often. When I use gel pens, I scribble a little on scrap paper to get the ink going. If I write on a surface that could clog the tip (like #4 and #5) I scribble again when I'm done writing.
#1: Textured cardstock. This paper has a linen-like finish. The texture showed through at least a bit with all three pens.
#2: Patterned scrapbook paper. This paper is smooth and all three of the pens seemed to work fine on it.
#3: Magazine paper. The Gelly Roll didn't do as well on this one - the line was a lot thinner than usual on this paper.
#4. Text page with acrylic paint. OK for all three. Sometimes on this kind of paper I need to stop often to scribble and get the ink flowing.
#5: Cardstock with colored pencil. None of the pens like writing on this waxy surface. I had to scribble a lot to get these words down. For this sample, I tried writing directly onto paper that had been colored with pencils. I've had more success writing on top of pencil that has been sprayed with workable fixatif.