XHTML 1.1 is the way to go ... it is the current standard since January 2000:
http://www.w3.org/TR/2000/WD-xhtml11-20000105/

If you *have* to include some older constructs, you will need to employ a XHTML transitional DTD (Doctype Definition), else I suggest using XHTML strict - it just forces you to write clean code, which will benefit you greatly in the long run.

Make sure you validate your pages:
http://validator.w3.org/

hth,
Alex