Unfortunately there isn’t just one. If there was one that trumped all others, all of us in the coatings industry would make to that one.
ASTM has identified the top 8. These report real moisture resistance, how it adheres, the ability to move with conditions, and ability to hold together rather than come apart when stressed, which is when you need your coating the most. Arguably the most important is how well your coating ages and is likely to perform down the road.
You can see how Topps stacks up at by clicking here. We also provide you stats of the typical 100% acrylic roof coatings for comparison.
Remember, a chain is only as good as its weakest link. Each physical property is a link.