If your boss has any basic science education try to sell them on the "a monoculture is at more risk to attack" approach. that's not entirely false, but mostly it sounds good and pointy-hairs tend to swallow it.
Then choose some version of Ubuntu or Red-Hat, but be ready to suffer all the horrors of dealing with the document, spreadsheet, calendar exchange formats. Those issues, more than any other, will spell failure. (just one middle-level moron who can't open your LibreOffice 'power-point' stack and you're toast) So, far more important than distribution is to be ready (practice!) your corporate compatibility two-step. (once saved my bacon by showing that my 'beamer' stack made everyone's powerpoint stack look like crap)
Beware of the vindictive IT staff who don't want to learn one more thing beyond their 'microsoft certification' merit badge. They will make your life a living hell. good luck!