What is the best quoting program to integrate with Keap Classic?
Pav
(Pav)
2
QuoteWerks? 
(Joking, it is me Eric from 10+ years ago.)
You may be better off posting your question in these Facebook Groups, as a lot of more Keap Users will see it and can back to you.
Otherwise, you could take a look at the Marketplace to see what 3rd Party companies offer in terms of API Integrations.
https://marketplace.keap.com/