Leeds Castle definitely!
This may sound strange, but I have seen a lot of castles all around Europe, and while I love the Leeds Castle site, I am more fond of the garden that leads from the parking lot up to the castle. It's about 20 minutes if you walk straight through, but I could spend hours looking at the trees, flowers, lake, small cottage, etc. If you are not into nature and gardens, there is a small shuttle/cart type of vehicle that can take you directly from the parking lot to the castle.
The castle is not disappointing at all!! It is still very nice. I just happen to remember the garden more.