I would always say the latest version, but I do not know enough about what you use Airwave for to advise you. There could be an issue in the latest Airwave that would stop you from doing what you currently do. I would email TAC letting them know all the features you use Airwave for specifically to see if there are any unreported issues. Please review the latest attached release notes (Current Version is 8.2.1.1) to see if any unresolved issues would stop you from upgrading. Make sure that you back up Airwave and download the current version that you are on before the upgrade so that you can revert if you need to.
There might be others on the forum that could give guidance about anything else that they have observed that might affect your upgrade, as well...