I agree with the above (mainly because I was with Thomsonru84 for one of the weekends).
Cli-Machx is worth a trip for the dh alone. There is a large fireroad climb at the start, but the singletrack is nice and flowy (my preference) and the dh is a monster!
Coed-y-Brenin wasn't so much my cup of tea. Seemed like lots of fire road for not a lot of singletrack and even some of that wasn't the best.
Llangdela was my favourite, but then I like smooth flowing singletrack over slow techy rocky stuff. I'm also fairly lazy, so the long but fairly shallow fireroad climb out to the back of the woods suited me fine. Then you ride back mainly on singletrack (which is great).
But as above, trail centres vary vastly and are a personal preference (although Whites Level is maybe my favourite of the ones i've ridden if you want to read anything in to that).