How about the raggamoffyn (sp?) monster: The PCs are called out to find a cure for the kings sickness. Ever since a while ago he has started to act funny and wear strange clothes... little did anyone suspect that the pink panty on the king's head
was actually a monster controlling him.... :swoon: the horror
Anyway I always liked oozes... mostly because I have fond memories of "The Blob". Could even make an interesting adventure... Ooze Cleric fed black pudding in sewers for several decades, now has turned into a gargantuan bonemeld (CR 20+). When the players enter the city sewers to investigate the numerous murders, they find out that the ooze has grown so large it's appendages spread through the entire sewer system.... not too bad a setup considering its about a creature called black pudding...
On a more serious note: intelligent monsters with vast resources and entrenched in powerful top-world organizations, I think, are more interesting than the typical monster sitting in its lair waiting for a party to kill it. So a dragon in a lair is certainly dangerous, but a dragon who was disguised as the royal advisor, and used his influence to send adventure parties after his rivals could be even more fearsome and memorable...
Same for a Illithid controlling the slavers guild or a beholder with his own cult following...
Someother ideas:
How about brain worms (don't remember the creatures name) taking control of the local wildlife? A Cranium rat hive taking control of the local thieves/beggars guild?