I had the same question some months ago, when I started to collect lego castle sets...
After reading and viewing sooo many reviews, posts, bricklink, brickset, brickipedia etc..
I think the right answer is "define your criteria"!
parts for MOC
my proposals are the first castles I ve bought in ebay:
simple solid realistic castle, not raised plate. I am thinking to buy a second in order to combine the two opened. Also, if you dont collect lion and crusaders you can sell-trade the minifigs in order to reduce the cost. (the complete set has 12 minifigs and 4 horses!!!). Also, you can build one without paying too much using bricklink and online instructions.
big, very good use of black and grey, very nice gatehouse and main tower, beautiful architecture and has also these awesome yellow walls. I think its the most beautiful lego castle. Its realistic and its has a variety in design.
small, elegant realistic castle (I will use it as an outpost.. its too small for a castle). I really love how it uses the yellow walls + black falcons minifigs, which I troop build
a very original castle-outpost-small keep with a dragon and a really beautiful design. not a traditional castle though
a huge and beatiful castle with lots of features.. you can use it not only for orcs, trolls etc, but also for the bad guys in a more realistic universe.. I can understand though those who have objections about its realism