Best version to upgrade to?

Hello, we are experiencing a bunch of little quirky goings-on since we upgraded from R5.0.8 to R6.5 (both servers and clients). I understand that these are probably bugs that hopefully have been addressed/fixed in further releases. From 6.5, where should we go? Some people have suggested 6.5.3, others 6.5.4.

TIA for any/all replies!

Subject: Best version to upgrade to?

We went from 5.0.8 to 6.5.3 then to 6.5.4 shortly that that. 6.5.4 cleared up Ambiguous Name message when attempting to address mail from the corporate directory.