Probably the height of the Eberron game I'm running at college was the battle with the Crazy Thorn Man. Let me explain.
The party, the Six From Sharn, includes in their number two rogues keen on pickpocketing. They attempted to snatch the purse of a gullible mark (in reality, the Dutchess of Karrnath, visiting Sharn), but were interrupted by a panicking child, who was babbling about being attacked by the Thorn Man, a horrible monster that turns little children into bushes, then eats them. Assuming the kid is crazy, the thieves manage to use the kid as a distraction and walk away with the Duchess' purse.
Flash forward a week or so in game-time. In order to get to Darguun, the same two rogues are stowing aboard the ship of one Ghaash Ratcatcher, a bugbear who does business in Korranberg in order to disguise his slave trading. The two rogues manage to convince the deck guard to let them "inspect" the ship, then go below decks. They enter a strange bunk, scattered with loot and strange thorny bushes, growing out of the wood.
"Hello", says the voice behind them.
It's a Thorn Man (the splinterwaif from MMIII). And the terror caused by that fight, as the rogues realized that their arrows were practically useless, and they called in the guard, who were instantly killed by sneak attacks, was priceless. As was the final encounter, as the party barbarian went downstairs for a one-on-one encounter with the cloaking beast, fighting around and through the ship's furniture until the splinterwaif succumbed to the barb's axe just as the barbarian collapsed as his rage expired.
It was a great series of fights, but I consider it a highlight because it's the only time I actually gave one of my players nightmares.
Demiurge out.