I went through all the options when i got my wagon built and ended up with carpet it doesnt get slippy even when it is wet but it is a little difficult to keep clean nothing that a stiff brush doesnt solve.
I upgraded the carpet on my last lorry ramp to these mats and found them absolutely fantastic. The little ridges mean that they are very slip free, the downside as far as I am concerned was that they are quite heavy and I'm not sure I have the payload to put them on my current ramp so they are just sitting around in the yard, waiting until I can get the lorry and kit over to a weighbridge.
The best stuff is the Kraiburg one piece rubber matting with all-in-one rubber slats built into the rubber. Earlswood supplies sell it, they advertise in Horse and Hound at back of mag.
I went for the same stuff used on Ifor trailer ramps with mine. Not cheap but very, very grippy and much lighter than some others, a bugger to sweep though!