Mostly vi. However, I use vim as my default editor in mutt. At one point, nvi (FreeBSD's vi) couldn't always do Japanese, and at the time I needed it more in email. And it became a habit. There's a couple of things I have set in my $HOME/.vimrc that I use much more in mutt than anything else.
Also, I do a daily online cryptoquote and vim has multiple undo, which I never learned to do in nvi. (Don't know if it's there). And going to the top of a file which I do with the cryptoquote) has become more of a habit in vim. I don't think it's necessarily the simplest move to top of file, for nvi shift+h works, but it's my habit. So, as mutt is one of my main terminal uses it's probably around 50/50.