Depends what you want. Big, restored castle full of sight-seers? Small, difficult to get to ruined keep that you can picnic in without seeing anyone else?
Kidwelly and Chepstow fall into the first. I’ll be going to one of the second category later. Haven’t decided which one, yet. 😉
Carreg Cennen is interesting. Dinefwr has the advantage of being NT and linked into a NT site, so other stuff to do. It’s also near other castles worth looking at.
If you’re near Gower you could visit Oystermouth, Pennard, Weobley, Oxwich and still have time for an icecream.
There are castles every 100m down here, so it depends…. 😀