Personally, I prefer XHTML 1.0 Transitional.
XHTML is XML based, so it allows easier parsing and you can also use the XML Components of most IDEs to programatically query and insert stuff.
Transitional is not as strict as strict, which makes it relatively easy to work with, compared to strict which can often be a PITA. Comparison between Transistional and Strict
1.0 is "more compatible" than 1.1 and 1.1 seems to be still under some sort of development.
