One rather useful piece of information you didn't include is what O/S you are currently using throughout the company (I'm making the assumption that the company is more than a few people). Unless there are significant problems with it, I would vote for using the same O/S on the production server. If you are doing it as part of a turn-key system for a customer, I would ask a similar question: what O/S do you typically use, and why not use the same one here?

All three are sufficiently reliable and secure for production use; all three are probably roughly equal in overall performance until specialized hardware starts coming into play (in which case, Sun will probably win; earlier versions of Solaris scaled pretty much linearly to several thousand processors)unless the company is severely cash-strapped, the initial purchase cost for software and hardware should not be a major factor. Support costs will be, which is why (I'm repeating myself) the O/S currently being used should be given first preference.

I've got a soft spot for FreeBSD, mostly because of how long it managed to hang on at Hotmail after Hotmail became a Microsoft property.