I live in North Wales and the trail centres etc are my back garden 🙂
opinions as follows:-
Coed Y Brenin- excellent trail centre, lovely area, for a weekend MTB jolly, cant see much better.
Llandelga- fun, but very manufactured and non technical (even the black sections) but real fun, whether its worth travelling 3 hours for and whether you’d want to spend 2 or 3 days riding there is questionable.
Penmachno- My personal favourite, rugged, challenging, ace, Its no better than Coed Y brenin without question probably not as good, but its a bit quieter and nearer (for me)
Marin trail (Llanrwst, Betws Y Coed)- Really good fun, not a whole days ride and not as technical as Penmachno and Coed Y Brenin, but a fun way to pass a couple of hours. Its so near to Penmachno that if you combined the two you could have a great weekends riding
That just leaves all the natural trails around the area, which to be honest put the trail centres in their place 🙂