XHTML 1.0 because it's more "backwards" compatible with HTML4. 1.1 has a lot of things you may not do that you might want to because you could in html4. But that's me... if I'm going to bother using XHTML, I'm using it strict. If something forces me to need to go to Transitional, I just drop down to HTML4 strict.
If your ban ever gets lifted, you'll want to read these FAQs from SitePoint: Frequently Asked Questions about HTML (Tommy Olsson) XHTML vs HTML FAQ (Tommy Olsson) XHTML 1.0 vs XHTML 1.1 (Kevin O'Brien and Tommy Olsson)