I think Naruto himself would be extremely powerful with the Omnitrix. His natural ability to learn new jutsu quickly would mean he could master using the Omnitrix's forms in no time. He could transform into aliens with unique elemental powers and combine them with his own Wind - style jutsu. For example, turning into an ice - based alien and creating a new type of Rasengan with ice shards in it.
Sasuke. He already has powerful Sharingan abilities. With the Omnitrix, he could add alien powers to his already formidable skills. He could use the speed of an alien form to enhance his own movement speed during battles and combine it with his Chidori attacks for even more devastating strikes.
Definitely Naruto himself. He has an unwavering will and determination. If he had a Pokemon like Charizard, their combined strength would be incredible. Naruto's ability to make friends and form strong bonds could also make his relationship with his Pokemon very deep. He could use his ninjutsu in combination with his Pokemon's attacks, for example, using his Rasengan while his Pokemon distracts the enemy.
In a fanfiction battle, it also depends on how the characters adapt to each other's worlds. For example, if Naruto learned how to play Yugioh and could use his ninja skills to quickly draw and play cards, he could be very powerful. Similarly, if a Yugioh character like Joey learned some ninja techniques, he could be a formidable opponent in the Naruto universe.
It could be Yubel from Yugioh GX. Yubel has incredible power and is very difficult to defeat. With her ability to possess and manipulate, she could be a tough opponent for Naruto characters.
Sasuke would also be very powerful. His Sharingan gives him enhanced perception and the ability to copy moves. In the SAO world, he could quickly analyze the fighting styles of opponents and come up with counter - strategies. His Chidori and other lightning - based jutsu could be very effective against the digital enemies in SAO.
I think it could also be a character like Itachi. His Sharingan gives him incredible combat abilities such as genjutsu and enhanced perception. If he were to get his hands on Halo technology, he could use it in very strategic ways. For instance, using a cloaking device to enhance his already stealthy nature from his ninja skills. And his intelligence would make him a force to be reckoned with in any 'naruto halo fanfiction' scenario.
It could be Master Chief if he got access to Naruto's chakra. He's already a super - soldier, and with chakra, he could become even more powerful. He could use chakra to enhance his already amazing physical abilities and his combat skills with the various Halo weapons.
It's likely Naruto himself. His ability to create multiple clones, his superhuman strength and speed from his training as a ninja, and his rasengan attack would be extremely effective against the walkers. He could easily take out large groups of them in no time.
However, we can't forget about Steve. He has been living in the Minecraft world for a long time. He has in - depth knowledge of how to build the most powerful weapons and structures. He can use his pickaxe with great skill, which could be as effective as a ninjutsu in some cases. Also, his ability to survive in the Minecraft world's harshest conditions gives him an edge over the Naruto characters who are new to this environment.
It could be Naruto in his Sage of Six Paths mode. He has incredible power, with the ability to fly, use various powerful jutsus, and his chakra is off the charts. In a fight against the characters from Toriko, his speed and strength would be very difficult to match.
In a naruto tekken crossover fanfiction, I think Orochimaru could be a very powerful character. He has the ability to use various snake - based jutsu and is a master of deception. He could potentially use his skills to outwit Tekken characters. But on the Tekken side, characters like Heihachi Mishima with his Mishima - style karate and his decades of fighting experience would also be a formidable force. It really depends on the environment and the nature of the encounter.