Sauron, if he were in his full power, would be terrifying. His ability to control minds and his dark magic could potentially subdue many of the 'Game of Thrones' characters. But he might underestimate the political cunning and the will of some of the more tenacious Westeros leaders, which could lead to his downfall in this fictional battle.
It's likely that Percy Jackson himself would be very powerful. His ability to control water gives him a huge advantage, especially in a world like Westeros where there are many bodies of water. He could drown his enemies or use water to deflect attacks. However, Daenerys with her dragons would also be a formidable opponent. Her dragons could breathe fire and fly, which is hard to combat.
It could be Jon Snow. If he has a Stand in the fanfiction, combined with his already proven leadership skills and combat abilities, he would be a force to be reckoned with. His ability to come back from the dead might also be related to his Stand powers in some way, making him even more formidable.
Klaus Mikaelson might also be considered the most powerful. His hybrid status gives him super strength, speed, and the ability to compel others. In the world of Game of Thrones, he could easily use his compulsion on many of the characters to get what he wants. He's also very difficult to kill, which would give him an edge over most characters from Game of Thrones.
It might be Ruby. Her speed and her scythe skills could make her a formidable opponent in the battlefields of Westeros. She could quickly move around and take out enemies before they even realize what's happening.
Well, it could also be argued that Daenerys Targaryen might be very powerful in such a fanfic. She has her dragons, which are extremely powerful weapons. If she were to enter Narnia, she could potentially use her dragons to gain a stronghold. However, she would still be no match for Aslan in terms of pure power and divine essence.
Perhaps Daenerys Targaryen. If she were to somehow acquire vampire - like abilities, she would be an even more formidable force. Her dragons combined with vampire powers such as enhanced senses and immortality (in a sense) would make her a very powerful and dangerous character. She could use her newfound powers to rule Westeros more effectively or to take on new enemies.
It could be the Scarlet Witch. Her reality - warping powers are off the charts. In a Game of Thrones setting, she could literally change the landscape, turn armies against each other, or even create illusions on a massive scale. She could make a whole castle disappear or make an army think they are being attacked by dragons when there are none. Her powers are not bound by the rules of Westeros' magic, so she could be extremely dangerous and powerful.
It could be Arya Stark. In Game of Thrones, she has proven to be a highly skilled assassin. In a Mortal Kombat crossover, her ability to change faces and sneak up on her enemies would be a great advantage. She could use her Needle sword and her skills learned from the Faceless Men to take on opponents. And her list of names of those she wants to kill could easily be added to with Mortal Kombat villains.
Well, some might argue that Aegon Targaryen with his dragon in Game of Thrones would be very powerful. But if we consider Fairy Tail, Acnologia, the dragon king, would be extremely powerful. If he were to enter the Game of Thrones world, he could easily destroy entire armies and fortresses with his dragon powers.
One great 'Lord of the Rings and Game of Thrones Fanfiction' is 'The Convergence of Westeros and Middle - earth'. It combines the magic systems of both in an interesting way, like how the One Ring could interact with the magic in Westeros.