LAMP is the best option because of easy availability and more webmasters are eager to work on that platform. Use windows, only if you have a site built on .NET .

Doesn't really which one you choose, although some will say one is better than the other. It really depends on what you are using to code your site. If you are mainly concerned with costs than linux/unix is better since it is in a more open source environment and windows is not.

I have sites hosted on Windows and Linux machines. I prefer Linux/Unix because it's so much easier to manage my site. I'd be lost without SSH.

It depends on the type of files that your site has - If they're ASP, go for Windows, If they're HTML, PHP, etc. go for Linux or Unix