The 1.5 mm calligraphy fountain pen (specifically a LAMY Joy with blue LAMY ink) has the most bleed through, and the medium point roller ball pen and the Pentel Pocket brush pen have a little bleed through in areas at the beginning and end of letter strokes. Gel pen (Pentel EnerGel in blue) has a little shadowing, but ballpoint (Uni-ball Jetstream), and pencil work the best and show no bleed through or shadowing.
LizThank Liz for uploading this and the other shots. This last one makes me think I'll pass on these otherwise interesting looking journals since I mostly write with a fountain pen.