I'm a huge fan of and have a developer license for ACF Pro.
That said, I needed a solution that I could include with free plugins that offered some of the cool features of Pro like repeating fields (aka Complex fields) and easy options pages.
There are a lot of frameworks out there, but I chose this one because it is one of the few that supports repeater/complex field groups well and is easily extensible. Good stuff!
FYI, if you want to pull it into the vendor folder via Composer, this worked for me: