I've been using Weber Howtex mostly. Because its what my tile shop recommends. Mostly using powdered stuff, so far very happy with it. Only problems I've encountered are the rapid set spf going off a bit too rapid for my slow noobie hands. Though its still good. Liked the Thick bed very much, as its a bit slower setting and can cope with building up uneven floor surfaces really well. Used it on my own floor at home inside and out, building up some areas by as much as 25mm. Worked a treat.
Have also used Ardex for a shower - my own - which was excellent but very expensive. I don't think I was getting any trade discount at the time though. The howtex is around £17 a bag which I think is really good for fully waterproof, flexible etc.
I've used BAL white star, which was great. But I don't think its waterproof (doesn't say so on the tin) so I don't have much call to use it.
As most of my work is wet room based and nearly always included u/f heating. I need fully flexible + waterproof stuff.
Paul