cjf is right about not getting an un-biased opinion. I would vote for FreeBSD, just cuz I like it :)

Your question should be more like, "What OS is best for this task?" since all OS's have strenghts and weakness.

One thing you did say that makes me suggest FreeBSD even more is your target machine. If you are starting from scratch anyway, build a machine with all the same versions of Apache, MySQL, Perl, and any other tools, that your site server has. This will make compatability issues minimal. It should reduce the "Well it worked on *my* machine!" symptoms as much as possible. And since FreeBSD is free, and fits all of your criteria...to me it is a given.

"Nothing is sure but death and taxes" I say combine the two and its death to all taxes!