It really depends on your purpose. HTML is more portable and "universal". It's easier for web crawler to navigate your site. However, if you really need to build a dynamic website such as this forum, you'll need to have scripting languages such as php and perl. I like php, because it is easy to learn, and can do the same job other script languages do.

I started learning .asp years ago but it looks like .php is the main one now. I would learn .php and mysql along with css.

webassist just announced they are not going to be working on .asp classic anymore and just concentrating on php/mysql. (I got an email recently from them about this)