I've had this question on my mind for quite a while now.

I see most email devs using the XHTML Transitional Doctype, with the reasoning of "it has the fewest quirks" or "it's more consistent"

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

I've been using the HTML5 standard Doctype for over a year now and I've personally never noticed any major quirks:

<!DOCTYPE html>

What Doctype do you use and why?
(Some examples of quirks would be great)