It completely depends on your target market. If you’re targeting small businesses in the US, Quickboooks Online should be your first priority. If you have a global market, Xero is probably second priority.

But first you should determine which ERPs in your target demographic have gaps that your software fills. Many of your features are already part of these ERPs you listed. You don’t want to have to build and maintain an integration for an ERP that already solves the problem you are trying to solve. This will require market research.

I do these exact kinds of integrations for a medium sized tech company (competitor of bill.com, stripe, coupa, etc.) and I can’t stress enough how challenging the ERP integration piece of the puzzle is. It’s truly a hard problem. The ERP space is vast, complicated, and constantly changing. And without an integration to a customer’s ERP, your software probably isn’t worth much to them. The one benefit is that all your competitors have this same problem.