Personally I think xcel is the best for me. A good amount of people I play with prefer Gamma Professional but only in 17g or 18g. It is surprisingly durable in both gauges especially for a multifilament. I also find that several people who prefer using Babolat VS Gut prefer Gamma Professional in their spare (rain) racket.
Its seems to me that NXT is preferred by players who are looking for a bit more control in a multi.
I have heard that Technifibre X-one biphase is popular.
Again they seem to prefer 17g and even 18g which is actually marketed as a squash string.