Katara would struggle most with Earth-bending. Earth nation citizens tend to strive for stability, security, and tradition. Their societies are generally conservative in nature and slow to accept change. Heck, even rebellious Toph became a cop and then later in life, preferred the ascetic seclusion of the spirit swamp. These patterns are congruent with the mindset needed to become good at earth-bending. Earth-benders need to project an unyielding spirit of the world moving around them because they are too resilient to be moved.
Katara was basically "screw stability" from S1E1. She was ready to leave her tribe with Aang the day she met him. Throughout the show, Katara is constantly shown to be a very liberal character in every aspect of her character. She drags her friends onto side-quests despite the team's protests about risk or time. Katara demonstrates moral ambiguity when she steals the water scroll from pirates and when she blood-bends the Fire-navy officer. She openly criticizes Northern Water Tribe's gender role traditions and challenges Pakku to a fight (fire-bending ambition).
Relationship-wise, Katara has the most difficulty connecting with Earth Nation people. She constantly fights with Toph, she is repulsed by Jet, and her inspirational speech to the Earth Kingdom prisoners failed to move them. Katara is also not a very materialistic person, a common hallmark of Earth nation people. She doesn't seem to care for attention, showing off, wealth, or even Sokka's science. (Airbending nature)
Sokka would struggle most with Airbending. Gluttonous for meat, skeptical to anything spiritual/mystical, and dismissive of anything outside of his perspective, Sokka lacks the psychical aptitude for Airbending. Heck, the boomerang is symbolic for someone that has issues of letting go. Airbending is all about detachment.
Sokka's innovative, creative, and strategic-mind would make him both a powerful water and earth bender. He has this unique combination of conviction and adaptability that great leaders are known for. This is most prevalent when he trains with swordmaster Pian Dao. Martial arts students are universally expected to be stoic, complaint, and conforming. Sokka stays 100% true to his clownish nature while still getting the job done. When he is instructed to do something against his nature, he finds another way to accomplish the task.
"There is more than one path up the mountain." Miyamoto Musashi.
This is why I confidently say that if Sokka manages to overcome his limitations and personality flaws to master Airbending, he would be the most powerful Avatar to exist. Imagine Airbending with Sokka's ingenuity.
Toph, airbending. Even if her sight was restored or she developed sonar, Toph would still be in an uphill battle against airbending. Toph grew up rich and hated it. Although Toph is not attached to the material aspect of her wealthy upbringing, she is VERY ATTACHED to her rejection of that lifestyle. She spends the entire series trying to prove to herself that she above it all, picking her nose, being rude, etc.
It is this very identity that drives her to be a great Earth-bender and the Godmother of metal-bending. However, this very attachment will impede Avatar Toph's progress in becoming an Airbender.
I have seen some people say, Toph's stubbornness will make her bad at all the other elements besides earth-bending. I disagree. Zuko and Katara were both very stubborn individuals. Stubbornness and the ability to accept change are not mutually exclusive. Change —the dominant philosophy of water bending— is something Toph is readily willing to embrace. I don't think Toph is as stubborn as people make her out to be. She adjusts her teaching style with Aang, she grew to like Appa, she apologizes to Katara, and she hugs Korra. Toph is not too stubborn to change, you just have to make it worth her while.
Also, the focus and ambition it requires to invent metal bending? I think that would translate well to Toph firebending.
Early Zuko? Waterbending. No balance. He'll be a danger to himself. He'd suck at Airbending too because he is so attached to "HONOR!" but I feel with Airbending, he just wouldn't be able to do it. With water bending, he'd freeze his own blood.
FireLord Zuko? Airbending. He is a Firelord. He has to be attached to the world. It's his duty to serve his country, not go on a personal spiritual journey.
Suki, firebending. She is a pretty down-to-earth and easy-going individual. She doesn't display any intense ambition. Then again, we don't get enough screentime with Suki to really know.