BCY-X without a doubt. I used (and liked) Dynaflight '97, 8125, 450+, and 8190.
We have two basic types of high performance material. 100% HMPE and HMPE blends.
Formerly, with the blends (450+, 452X) durability was decreased and colors weren't as bright. With 100% HMPE, it's not as stable, especially in higher temperatures.
X is the best of both worlds. Very stable, very durable, great color retention, quiet...doesn't get any better.
The others I mentioned are still good materials--the X is just better.