It's hard to say for sure, but Anakin Skywalker could be extremely powerful. His natural Force abilities combined with the power of bending, say firebending, would make him a formidable opponent. He could use the Force to enhance his bending skills and his lightsaber combat skills.
Yoda might also be a top contender. His deep understanding of the Force in Star Wars could be applied to bending if he were in the Avatar universe. He could potentially master bending very quickly due to his wisdom and connection to the Force. His small stature but great power would be an interesting addition to the bending world, and he could teach other benders new ways to use their powers, perhaps even introducing a new form of bending that combines the Force and traditional bending.
Perhaps Naruto combined with Toph. Naruto's agility and combat skills along with Toph's earthbending mastery would be quite a combination. Toph could sense enemies from afar using her earthbending senses and Naruto could quickly engage them in combat. Also, Naruto could use his clones to help Toph with larger - scale earthbending projects, like building massive structures for defense or offense in a battle.
I think Qrow could be really powerful in this crossover. He has his own unique abilities in RWBY, and if he were to also master the Force, especially the more combat - oriented aspects like telekinesis to throw his weapon with more force or use the Force to enhance his reflexes, he'd be a force to be reckoned with. He also has a lot of experience in combat which would help him adapt to the new powers.
Anakin Skywalker and Aang are quite popular. Anakin's journey from a young Jedi to Darth Vader could be paralleled with Aang's growth as the Avatar. They both have a lot of power and face internal struggles. Also, Luke Skywalker and Korra could be an interesting crossover. Luke's hero's journey could be compared to Korra's as she overcomes various challenges in her world.
It's hard to say for sure, but Master Chief could be a strong contender. His combat training and advanced armor give him an edge. However, Darth Sidious is extremely powerful with his dark side abilities. He could potentially manipulate Master Chief if he got the chance. But then again, the Forerunners from Halo have such advanced technology that they might be able to counter the Force -powers. So it really depends on how the story is written.
Zuko is extremely popular as well. His complex character arc, from being a villain - like figure to a hero, is very engaging. People love to see his transformation and how he overcomes his inner demons and the expectations of his family and nation.
It's hard to say, but Sasuke could be a really powerful character in such a crossover. His Sharingan abilities combined with possible bending skills could make him a formidable force. He could use the Sharingan to predict an opponent's bending moves and counter them effectively.
Another possibility is Doctor Strange. His magic is already very powerful. If he were to gain some of the unique abilities from Ben's aliens, like the XLR8's super speed for his spell - casting, it would make him even more formidable. The power balance really depends on how the fanfiction writer chooses to blend the two universes' powers.
A crossover could also involve the political intrigue of both universes. The Galactic Republic's complex political system could be compared to the different kingdoms in Avatar. Characters like Padmé Amidala could interact with Avatar characters like Katara, and they could learn from each other's experiences in dealing with political unrest and power struggles.
The idea of redemption also shows up a lot. There are characters in both Avatar and Star Wars who have made mistakes or turned to the dark side. Fanfiction often explores how these characters can find redemption, perhaps with the help of characters from the other universe. For example, a Sith Lord might be redeemed with the guidance of an Airbender. This creates interesting character - development arcs.
Sakura from Naruto is often included in these crossovers as well. Her skills as a healer and her growth as a strong female character can add depth to the crossover stories. She could potentially learn new forms of healing from the Avatar world or teach her own medical ninjutsu to Avatar characters.