There are relatively few truly open source ERP systems that are on the market. The 2-3 among them are OfBix (maintained by Apache Foundation), Adempiere derivatives (with no home) & ERPNext maintained by ERPNext foundation & Frappe
Ofbiz is Java-based
I have tried to use OfBiz since 2002, but I have always found it really complex as the screens are relatively overcrowded and feel dated. Any customization also needs JAVA programmers which were always a bit scary for me from a business perspective. But if you have serious JAVA developers in the house or can find experienced OfBiz developers and you have deep pockets, this could be an option
Adempiere Has No Home
Adempiere, an open-source fork of Compiere has been on the market for > 10 yrs now. However there are really no consolidated efforts except some companies here and there. Again based on JAVA, it could be an option. I cannot say what the UI/UX looks like as it is hard to find. It would be an option if you have a lot of time to evaluate & deep pockets.
Odoo used to be open, not anymore
Odoo used to be my favorite until the went from “OpenERP” to “ClosedOdoo”. A hosted option can cost you quite a bit. However, the design is modern & relatively user friendly. However, the decision to subscribe by modules/plugins can be painful and expensive.
Is it the best ERP system?
The answer can be found in the Gartner rating for 2017, 2018, and 2019, where it was rated by its user in the Top 5 in the “Frontrunner’s Quadrant”.