Between the two, Afan. The Gap is great to look at, but a bit dull as a ride and seems to go on forever. Also, if you use the Taff trail to get there from Merthyr, it’s a bit like riding through an everlasting rubbish tip. The Gap would be better as a walk. Indeed probably stunning. Afan can be fun, although it does love a fire road. FOD is probably the best out of the three – less of a slog to get around (the red is a bit dull) and you can ride the good bits over and over again without spending hours getting to them. You also feel part of the gang – it’s small, so you are always riding with somebody.
In the spirit of STW, this weather, I suggest do a big thirty mile loop of Exmoor out of Minehead (avoiding Minehead itself!).