I suppose that depends on how soon you want D7 and how much you're willing to do the D7 conversion. When I looked some time ago, casetracker at the time, was my choice. If IIRC, it started with the code from project issue and added to it. Project and project issue are of course what the d.o/project/issues pages are using.
I also think casetracker looks best.
Thank you, Fred