Dyna Engineering Ltd. 
dynaengineering.com
Incorporated: 1971
I really like how Dyna has been around for a long time, it shows they know what they're doing if they've been around so long. They were a tool and die making company (still not sure exactly what that is lol, but it sounds smart!)
Top 3 reasons I'd buy from them:
1) Their overall offering is super cool as they have hurricane resistance and wind load information on their sign brackets.
2) They have a wide range of modular and universal post brackets that makes it really easy to pair what you need together for most jobs.
3) Their prices are unbeatable which makes their overall value astonishing!
Top concerns about buying from them: -
At first I was concerned that their prices were the lowest I'd seen in a long time so I wasn't sure about their quality but with the testing videos they sent to me, they were able to prove the "engineering" in their name!
Just about everyone else also had some sign brackets that were unique as well so of course it's hard to source absolutely everything from one place.
They took a little bit longer to ship product to me but overall the savings makes it worthwhile.