Llandegla’s good fun and closest. If you’ve got the whole day then get up early, drive there, do a lap, and drive back via Cannock, makes all the driving seem a bit more worthwhile if you do two trails in different places in a day. Personlay I’d make a weekend of it.
Sat am – Cannock
Sat pm – Llandegla
Sun am – Snowdon (off the hill before 10am rememeber)
Sun Pm – Coed-y-Brenin (I’ve never been, but it seems the obvious one after doing Snowdon)
[edit]
Or, a very early start, drive to Stainburn (short laps but very techy, probably as much tech as a 30km trail center black run in just a few miles) even if you do all three loops you could be in and out in 90min. Then over the other side to do Guisburn or Lee Quary. Folowed by Llandegla, probably best taking lights though!