In reply to Red Rover:
Carnedd y Filiast has a small layby that can fit maybe 4 cars. It usually isn't full in my experience but would be worth going early.
I always find Cadair Idris quieter than the main part of Snowdonia. There are usually people on the Cyfwry Arete, but even that usually isn't exactly crowded. Cyfwry Arete and Table Direct would be the obvious choices there.
If you can stretch to severe and don't mind walking then the S ridge of Rhinog Fach is excellent.
If you really, really don't mind walking then Christmas Retreat in the Arans gets you the "arguably most remote route in Wales" tick at vdiff. It's got some good pitches but is a bit of a contrived line, so probably only worth it if you really want to escape the crowds.