i have tried windows 2003 and ive tried several linux i wld pob stick to linux though im noob at it my favourit operating system i like to use for server wld be the wierdest 1 i love using windows xp pro i run my web server, mail server, team speak server, ventrillo server and a few game servers off windows xp pro over a d-link router
I'm now using Windows Enterprise Server 2008 x64. Because of an MSDN subscription I have access to a lot of software from Microsoft.
BTW, I use IPCop for my router/firewall on a decently powered PC. It is fairly easy to install and configure plus I have far better control over it than a commercial "home" router.
I don't trust myself with my own server... yet. I stick with free server services to do what I need. If I did choose to manage my own, then I would dive right into Linux.
I have debian, because it is more stable than ubuntu, which I'm also familiar with. Also, it's the first Linux operating system I ever used on a server (before I had Windows (XP, 2000, 2003)).
Soon, I will buy OS X 10.6 Server (not buying 10.5 since there's no point anymore) and install it on my mac mini.
So - Current - Debian, Future - OS X Snow Leopard Server.