The best plastic Tyraps (registered trademark of Thomas & Betts) are still the best, with the little stainless insert grip. The black ones hold up extremely well in direct sunlight. Note that T&B bought another company that sells to Home Depot under the T&B name but not Tyrap, and those are junk.
The benefit of the stainless ties that Denise mentioned is that they have a MUCH higher tensile strength than any plastic strap, and they don't elongate and loosen over time. The UV resistance is just a lucky coincidence!
Gary H. Lucas