Best CRM to build apps for?

I build apps for platforms like Slack/Airtable/etc, and have considered building some for a CRM. I know Hubspot/Salesforce are the biggest, but I'm curious what users of various CRMs think are the best products to build on top of?

I'm specifically interested in which ones you think have a lot of unmet potential (or rough edges) that you wish there were more apps for?