Bear in mind that just because ArchiCAD can 'open' a file, the editing experience won't be the same as if you had opened it in the software that created it. There may/will be some information lost in translation. This will be the case regardless of what software you decide to use.
However, IFC is usually the best format for 3d BIM models and generally will accurately bring in the geometry and any data held in the geometry. I'm unsure of how well it deals with any 2d linework/text that may have been added manually to a 3d model; you may have to experiment. There is some more general info here - http://helpcenter.graphisoft.com/guides/archicad-19-int-reference-guide/interoperability-2/
And just to avoid confusion, BIMx isn't used for transferring data between BIM programs. It's a self contained presentation of a 3d model with optional read-only 2d drawings all contained within one file. It can't be opened back into ArchiCAD (or any other BIM software).
If you want to check how ArchiCAD opens certain file types, you can download a demo or a fully functioning trial version from - https://myarchicad.com/