Diamond plate would be a slick as the door itself. There's more flat surface than there is raised on the diamond plate. The best in my opinion would be expanded metal, more traction surfaces and very little flat surfaces and areas to displace water.
Or, like me, leave it at home and park it in the garden shed. I've had my one and only spill on that ramp.
2013 Voltage 3800, 2012 Chevy 3500 HD
2010 Yamaha V Star 950
2009 Yamaha Raider
Volunteering with Habitat for Humanity Caravanners