It depends, are you looking to breed or just for a pet?
Watch for spinal deformities or odd kinks in the spine. Look at the scales, are they all in uniform rows or crooked/jagged near the tail base. Mouth, look for a good non crooked mouth that isn't pinched. There are many factors, if your looking online, then I will do a more indepth description. If your buying from a store then get what appeals to you, imo, there is no high quality store betta. If your breeding, online is the way to go.
I would also say that as far as colors go, there is no rarity. You may find common colors, but really there arnt any rare.
Heres what one common color is
Teal dragon with red wash. If your looking to breed and you don't want red offspring, avoid the red wash.
Theres also Solid whites/platinum. Gold, black, orchid, various types of copper. Red, black, normal coppers. Mustardgas is common now aswell, though I will say this now, there is no mustardgas on the market. Just fish that look like them. MG was a line name, not a color morph . Unfortunetly, that line is basicly gone from the betta world, I have seen 1 though, a male but he was too old to breed. I have a steel Dragon "Mustardgas" aswell.
Basically, what's considered mustard gas is any betta with a blue body, orange/yellow fins, and black or blue edging. The true MG line was amazing, They had purple iridescence, yellow fins with orange and red swirls/streaks and black edging with more purple.
Yes, I was looking online at a place called bettaakapes.com Have you ever heard of them?
Chard56 is a good seller, breeds his own fish too. I've been talking to him about ordering one of his HMPK's he's been great! Also uses the forum a bit.
Can you find me a perfect HMPK in the US that's under $20?
...flat edge on his tail . Jkjk, love him. Hes beautiful. If you want a quality betta, pay more then $20. You want cheap genetics, by all means. If and when I get my next betta I will be doing so via aquabid. I exspect to pay atleast 25-40 for it.
So kim, link us?