99.99% of the time new web pages should use the following doctype.
The other 0.01% of the time you'd use
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
Unless you know for certain that your page requires the second of these you should use the first.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
The transitional doctypes are for old web pages that you haven't had time to fix properly yet. You'd only use them thr new pages added to a site using a template where the template hasn't been fixed yet.
You can't use XHTML 1.1 for web pages at all since Internet Explorer doesn't understand XHTML and XHTML 1.0 is the only XHTML version that can be served as HTML.