Hmm, let's start with some criteria:
Sets built of bricks = better
Sets highly dependent on panel walls = worse
Sets highly dependent on CRAPP = worst (CRAPP = Crummy Ramp and Pit Plate) (though I'd say raised baseplates in general)
As far as figs go:
Outlaws = cool!
Knights = great, but not as great as outlaws
Willa the Witch = lamest fig ever
So, mixing those, I'd say my favorites tend to be the outlaw sets, and I'd go with:
Wolfpack Tower
Forestmen's River Fortress
Dark Forest Fortress
Then knights:
Black Falcon's Fortress
King's Castle
Knight's Castle
Royal Knight's Castle
Fire Breathing Fortress
Finally, the Willa effect:
Night Lord's Castle
That said, these are all awesome sets that I'd love to get. Even the Fright Knights have their place in the history of Classic-Castle. That set, for instance, has 48(!) 1X2 gray bricks, not to mention 27 black plus a few in other colors, and then numbers of 1x3, 1x4, 1x6 etc bricks. Also good numbers of plates, arches etc. How many sets today have that many basic bricks? And, as much as I bag on Willa and also the silly bat helms, Fright Knights sets are otherwise stocked with pretty useful fig parts, including my favorite torso:

Plus good old pieces like the bullet helm, the helm with neck protector, old short sword, old great sword, halberd, bow etc etc etc.
Bruce