HTML or XHTML?
For a nice explanation of the differences between HTML and XHTML, as well as recommendations on when to use what and why, check out the October 2003 issue of WaSP Asks the W3C.
So, what do I use? It depends. For this site I use XHTML 1.0 Strict, since I am the only author here and have full control of the code. For client sites I mostly choose HTML 4.01 Transitional. The reason being that most clients want to edit their own content which almost always means that there will be one or several authors with (too) little HTML knowledge. Plus there's also usually some kind of junk-code-creating CM tool involved. By using HTML 4.01 Transitional there's at least some chance that the content added by the client will validate.