StarUML is not open source (it was), so I would exclude it
UML designer is abandoned and replaced by Papyrus Web which, looking at the POM.xml, is based on XMI 2.5.1 (UML 2.5.0)
Papyrus is based on XMI 2.5.1
Modelio is based on XMI 2.4.1
Gaphor is based on XMI 2.1
As the current parser is based on XMI 2.5.1 of Enterprise Architect, I would start with the one of Papyrus/Papyrus web backed by Eclipse Foundation, then, if needed, adding the others.
The standard is XMI and there are different UML tools implementing it such as:
See https://github.com/ModelioOpenSource/Modelio/issues/74