
It might not be a big castle, but the design is perfect.
Second favorite castle set would have to be 6086 Black Knight's Castle, it's probably the "best" castle and castle set there is, which would answer the topic question.

As a kid I always wanted 6059 Knight's Stronghold, especially since the Black Falcons were being phased out by the time I was old enough for Legos.

While it's not so high up on my list now I REALLY like the Falcon knight that comes with it.
I also really really love the front of King's Castle. The entrance is so awesome. I like the fact that it's got four high walls too, but the rest is kinda boring and it's just castle wall pieces. Limiting it from my top 3. But I thought I'd mention it anyway '(because) it's good.