I would say clearly chrome for v6.1.
For v7.0 probably too.
It seems OpenERP developper and support teams are mainly doing there tests on chrome, chromium and firefox. Certainly because most of them uses Ubuntu being in the Open Source spirit and those browser are more accessible on it.
On v6.1 we had some issues with Firefox, it's much more stable on v7.0[1]
For other browsers I haven't experienced much of it.
However, you can track errors on
openerp-web bug tracking
You will see that users makes bug reports for specific browsers and OpenERP is constantly improved.
Thought, I can only suggest to avoid Internet Explorer but this is mainly my opinion.
[1] : lp: 1029311
We use firefox and chrome without problems. Never used (and never will) Explorer