If you’re doing the Beast, study the map before heading out in order to avoid a big pointless fire road loop.
Nant yr Arian is my favourite, lovely flowy trails and if you want a long ride there’s a big semi-natural feeling loop too. Coed y Brenin is great and has loads of rocks. Penmachno is very good too and has a more mellow feel, more traditional XC than some of the others maybe. Cli-MachX at Machynlleth isn’t long but has a great finish. It’s been too long since I last rode at Betws y Coed for me to comment. Llandegla seemed like a slightly poor pastiche of the red at Glentress and was a bit of a disappointment.