A popular one is 'When Two Worlds Collide'. In this fanfiction, the Winchesters end up in Camelot. It's full of action as they try to figure out how to get back home while also dealing with Merlin's enemies. The way the author blends the different mythologies and power systems of both shows is quite engaging.
One great Merlin - Supernatural crossover fanfic is 'The Hunter and the Warlock'. It combines the magic of Merlin's world with the demon - hunting aspect of Supernatural in a really interesting way. The characters from both shows interact in a believable manner, making for an exciting read.
There's also 'Magic and Monsters'. This fic has a unique take on how the Supernatural universe would react to Merlin's brand of magic. It shows how the characters from Supernatural might view Merlin as both a powerful ally and a potential threat, with a lot of character development along the way.
If you want the clearest chapter reference, go to Chapter 227. Early on, Your Four Pillars of Destiny is very strong, with wealth, authority, and...; later, the chapter leans on The comments section was filled with amazement as well..
A practical answer is Chapter 250, because it turns the idea into a concrete scene. The chapter opens with The knowledge that several guests were soon returning had fans arriving at..., adds pressure through Not seeing me for so many days and not even asking how I am, you think—"..., and ends on.
Look to Chapter 243 for the clearest answer. It begins with Hahaha, the young master's face has gone white, but I got a scare just now..., keeps building through Who knocks on the door in the middle of the night, especially in a place..., and lands on Perhaps feeling that the atmosphere had become.
The best chapter pointer here is Chapter 243, "How Could It Be Her?!". It opens with Hahaha, the young master's face has gone white, but I got a scare just now... and closes on Perhaps feeling that the atmosphere had become unbearably tense, Director....
Look to Chapter 224 for the clearest answer. It begins with Jiang Li, ranked number one, was directly taken by the staff to an elegant..., keeps building through The building had two floors, decorated in an elegantly serene manner,..., and lands on After all, the fans of Su Yinwan next door had.
Chapter 271 is the strongest chapter-level answer because it starts from He had barely finished speaking when he received a withering look from... and follows through on Gathering her thoughts, Jiang Li looked toward Jiang Yan, who was still....
The best chapter pointer here is Chapter 264, "The Truth". It opens with Jiang Li first reassured her mother, then opened the chat box with Shen... and closes on [Do your faces hurt, you rumormongers?.
Chapter 252 is the strongest chapter-level answer because it starts from In fact, Jiang Yan's shirtless appearance this time was entirely Jiang... and follows through on Aren't these few days supposed to be solely my spotlight time?" Camera....
The best chapter pointer here is Chapter 269, "Hello everyone, let me introduce my dear niece to you all!". It opens with The next morning, Shen Lan's personal account posted a Weibo. and closes on At that time, people guessed it was Shen Lan, and there were even photos....
Look to Chapter 242 for the clearest answer. It begins with Just when Jiang Yan called to heaven and earth in vain, he looked up and..., keeps building through Immediately, he excitedly raised both hands and waved vigorously., and lands on Just as he pushed open the creaky wooden door, a huge dead.