You want to use the one that describes the type of HTML or XHTML that you are using--and the type that you are using will depend upon what it is that you are doing. Strictly speaking, we were supposed to stop using the transitionals sometime back in the last century. XHTML isn't as universally acceptable as HTML so on the internet go with HTML 4.01 Strict. For local use I've been using XHTML 1.0 Strict.
“The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect.”
—Tim Berners-Lee, W3C Director and inventor of the World Wide Web