Of the few I've driven; Castle Combe, Donington Park, Silverstone and Pembrey I'd say hands down without a doubt Castle Combe is my favourite.
It is however the only circuit that I have been on multiple times (double figures) in a range of vehicles including my old mans TVR Cerbera and a Formula Ford. I also drove on it back in the 90's before the "safety revisions" where made on it so I do remember how fast it was before they added the two chicanes (The Esses & Bobbies).
Also when I did Silverstone it was in a Lotus Elise and I wasn't able to do the full F1 circuit as there was other events going on in other parts so did a shortened version of the track. Definitely need to get out more on different circuits.