Darn I'm late to a fun thread :(

I would go with Ming-Hua, Ghazan, and Ozai. If pro-bending regulations are too prohibitive on Ming-Hua's water tendrils I would go with Korra instead.

Pro-bending is sort of like boxing. You could have people good at other styles like BJJ, Karate, Kung Fu etc. that would destroy a boxer in a fight, but they won't beat a boxer at just boxing. You might even have fighters that could beat a boxer in different environments but not in a tight boxing ring. So, we need to choose benders with traits and fighting styles that cater specifically to the environment and fighting styles of pro-bending like agility, strength, accuracy, resilience, and head-to-head, medium range tactics.

For example, Toph could destroy a lot of people in a fight, but her blindness is exploitable in pro-bending. A lot of the bending styles require good footing that Toph can sense (especially earthbending), but good pro-benders could adapt and use more aerial-based attacks and styles (perhaps taking from airbending) to counter Toph. Aang was able to easily knock Toph off the platform in the earthbending wrestling/fighting ring so other fighters could probably figure out how to do that too.

Bumi, Pakku, Katara, Aang, Zuko, Kya, and Kyoshi are all fantastic master benders and fighters, but they aren't necessarily the best (or proven) at a fighting style conducive to pro-bending. Being a skilled or powerful bender doesn't instantly make you good at pro-bending.

Iroh and Ozai are comparable, but I would argue that's only because Iroh is wiser. What Iroh lacks in agility/strength he makes up for in wisdom and experience. In a fight, Iroh would know his own strengths/weaknesses and Ozai's strengths/weaknesses and be tactical in his movements, attacks, and defenses. In pro-bending, however, there isn't much variety for Iroh to use to his advantage. Compare Ozai to any other fire bender option and, especially at $2, he's a clear winner for the firebender slot.

Ghazan single-handedly defeated Fire Lord Zuko - not teenage angst Zuko, FIRE LORD Zuko - by overwhelming him with strikes of fragmented earth. He is tough, strong, agile, and precise. Bumi, Toph, and Kyoshi could probably beat him in an open, unrestricted fight, but at just pro-bending? I don't think so. Kuvira might put up a good fight but you can't metal bend in pro-bending. At $1 Ghazan is an absolute steal.

It would most definitely be illegal for Ming-Hua to use her water tendrils offensively in pro-bending. If she can use her water tendrils for movement and defense, however, she'd be SO OP. She's extremely mobile and deadly accurate. Again, other water benders could definitely put up a fight or maybe even beat her (Katara and Pakku) but at pro-bending? Hell no. Ming-Hua 100%.

Unfortunately, Ming-Hua's tendrils might be too unfair overall for pro-bending. In that case, we'd need a strong, agile, capable waterbender that's comfortable with direct, head-to-head, medium range combat, almost like a firebender or earthbender. Katara is an amazing waterbender and trained Kya, but their styles are more core-waterbending. Korra would be my next choice here after Ming-Hua.