Skingrad's castle looks coolest from the outside, way up on that hill. But inside, it's rather small and undramatic.
I've always loved Bruma's castle on the inside, but really I'm torn between Bruma, Chorrol, and Cheydinhal. They're all cavernous, majestic, and with lots of areas to explore (if I'm tresspassing, lol).
Strongest castle in battle? Probably Skingrad. Would be pretty tough to get a siege on it, the enemies would have to cross that bridge somehow while managing to stay
on the bridge.