Holy cow, I can't believe I still have an account here!
Anyway, as difficult a decision as this is, I have to choose the following three:
#1 6079 Dark Forest Fortress - I remember my excitement upon winning this set on eBay awhile ago. It has the perfect mix of the forestmen's charm and element of fantasy with excitement and playability, without going over the top. Also, the minifigs are just brilliant.
#2 6071 Forestmen's Crossing - I'm still amazed that I have this set. I got it, along with King's Mountain Fortress, and other various classic castle stuff, mixed in a box from my cousin when I was very little, when he decided he didn't want it any more. One of this sets best features is it's baseplate. Of course, who could forget that Forestwoman (who I've regrettably lost...

)?
#3 6087 Witch's Magic Manor - This one is often on peoples' least favorite set lists, but if I didn't put it on here, a child version of myself might show up and kick me. As far as I can remember, this was my first Castle set, and I gained an absolute love of witches because of it (I'll still defend poor Willa as one of my favorites). To me, it has this really strange charm I can't really explain or justify, because the set really is pretty ugly.