I'll list as many as I can, I'm really looking for opinions on this.
It's a Big deal for someone wanting a legal buisness in growing. If I didn't mention the one please post and tell me why you choose them. That said, please CHOOSE YOUR FAVORITE.
Have A Good One
The people actually doing the business in growing (legal or otherwise) generally aren't starting from commercial ceeds.
Instead they usually start from clones, often from "elite" clone only lines.
If you look at the dispensaries doing the legal SELLING much if not most of what they're selling is buds grown from "clone only" lines, not from commercial ceed.
So, I dispute the premise of your question. You really don't need to know the best "breeders" to establish a legal business in growing. You just need to identify the best individual strains for your particular niche and grow setup then source them. Who bred them or the quality of the rest of their work is secondary.
For example, if one breeder offers 8-10 crap strains and 1-2 great ones, shouldn't you consider the 1-2 good lines if they otherwise meet your needs? I think there are breeders like this out there.
To answer the question anyway, I'd say the most reputable breeders include:
Mr. Nice (Shantibaba), DJ Short, DNA Genetics, and Serious Seeds. These are all outfits with outstanding individual lines that have been around for a long time. (DNA, the shortest of the bunch, but still good). Some "upstarts" with good reputations include Rare Dankness, Gage Green, Bodhi, Sannie, TGA. Like anything, it comes down to what exactly you're looking for. Not every strain from each of these breeders will be a "winner", but they'll have some good ones or good phenos to pick from.
Sensi was once the best seedbank/breeder, but I don't think they've really kept up with the times, and a lot of people think their lines simply aren't as good as they used to be. Dinafem is more of a large commercial outfit, though they do offer high consistency and good product.