Our main web server runs Debian, with apache2 and php5 on top. This is now happily serving automatic banner adverts for hundreds of online shopping carts, and over 4000 images are served each day. All very reliably!
There is no other sensible operating system choice for a web server. Debian is very stable, simple to update and it has a fantastic user base on the Internet so that any problems encountered are simple to fix. Setup was a breeze!