Tried gel, wood, and leather.
Gel: feels OK at first, but a bit too "grabby". Quickly becomes disgusting, then starts to leak gel and becomes doubly disgusting, and useless shortly thereafter.
Wood: too hard, not comfortable.
Leather: by far the best. As with all leather products, quality is important. I use EK's "3-Star" wrist rest, which has held up well after years of use.