The best one I've ever owned is made of "Sunbrella" fabric. Get one made for your specific type of boat as close to the actual length and width as you can.
I always get a custom cover and tell them will use for trailering. This way they will add padding anywhere it might hit s cleat or windshield corners.<br /><br />SunBrella only way to go, it has a 5 year warranty and both I have had lasted much longer.