Apache is pretty much the standard for a lot of companies so you can't go wrong with learning that. But you should def. do some reading on nginx, and lighttpd as well
Nothing in particular, just read up on how lighttpd and nginx differ from Apache, and why one or the other is better suited for different tasks.
For example, Apache has a lot more modules that can do obscure things that aren't mainstream for a web server. Such as LDAP, and WebDAV, these modules might make Apache better suited for an intranet setup. Even if the nginx community popped out a good WebDAV module, the Apache one would still be a better choice because of the vast documentation and larger user base and community support already available for it.