HTML 4.01. There is absolutely no reason to use XHTML for anything but experimental or academic problems that you only want to run on the 'obscure' web browsers.
XHTML Transitional is completely pointless even to those browsers, so I'm not sure why anyone would aim for that. It's actually pretty alarming that a number of people would recommend that.
I'd say aiming for HTML 4.01 is the most predictable, but Teifion is right really, "anything that renders your page will do".
in response to Michael Stum:
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.
This is certainly not true. A lot of XHTML on the web (if not most) does not conform to XML validity (and it needn't - it's not being sent as XML). Trying to treat this like XML when dealing with it is just going to earn you a lot of headaches. This page on Stack Overflow, for instance, will generate errors with many unforgiving XML tools for having invalid mark-up.
