They seem to really like the Uni-Ball pens, which I've used a lot and are OK, but I've switched very happily to the Inc R2 Rollerball [1]. Really dark and even line, very light pressure, and low friction. Just a very comfortable pen, cheap enough to have around the house by the dozen but feels like a high-quality pen.

[1] https://www.amazon.com/ROLLERBALL-Comfort-Flowing-Writing-Pr...