This is an excellent question. Here is my experience based on purchases I have made from these sellers.
HMPK, bar none seller Ninebettas. Here is Sathit’s website http://www.chonburi-betta-lover-club.com/
His betta’s fins are superb, really bright colors and they are extremely healthy. His customer service is excellent. And you can contact him to make a special request; you don't have to wait until he post on AB. He has bred one of the best super black HMPK I have ever seen. I have a standing order with him, waiting on that special super black pair. Sometimes he gives me sneak peeks, and I have swooped up a few fish before he has posted them on AB.
HM, this is a hard one. I would argue that in terms of uniqueness and color range, Banleangbettas and Aquastar71. I rate Aquastart71 customer service much higher than Banleangbettas, and Kit really works with you. My only complaint is that AQ71 betta's hate to be shipped and in my experience end up being tail biters. I spent $81 bucks on one of his fish and never got to see him in his full glory because he bit his tail off during shipment. On the plus side, he has turned out to be an excellent breeder.
Banleangbettas, definitely sells some of the most unique fish I have ever seen. But their fins can leave a lot to be desired. But I have never seen red dragons like his. Very clean bodies, but the fins need work.
For marbles, Ploybettas, there is just no comparison. I have posted a thread on some marbles I got from him and I have never witnessed such a magnificent transformation. I was also very happy with the customer service.
In terms of HM finnage, Interbettas has to be my top choice. Sanya consistently breeds high quality betta’s, has excellent customer service and if you buy a pair from him, he will give you his opinion on which female will be the best match for your male to achieve your desired goals. I love working with Sanya, but I’m not really fond of coppers and that seems to be his main focus now.
One seller I hope to purchase from one day is Nicebetta's. I really love their style, but I have not seen them on AB in awhile. They breed a fish labeled Serenade or something like that.
A little long winded, but hopefully this helps. Here are some pictures of my fish I referred to throughout this thread.
This is Royce. I am in love with this man. Great personality, very healthy. Just makes me smile thinking about him.
Here is Brick from AQ71, beautiful, but he will bite his tail when he is stressed.
Here is Troy, from Banleangbettas. Just look at his body, nice clean scales. I had to have him even though his fins need a lot of work. I didn't buy with the intentions of breeding him. But he is a great fish.
This is Benjamin from Interbettas. No complaints, and his fins are amazing. He is in the breeding tank right now, I'm hoping for some fry in a few days. Out of all my HM he has the best fins. And he is funny.