Vi. The simplicity, speed, and the fact that it is installed on any *nix system by default are my favorite aspects of Vi. I've played around with Vim, but haven't migrated to it because I just don't use the features.

At times, I have been tempted to move to Emacs due to all its cool features but have not yet, maybe some day. Emacs in slime mode has been great to use while learning lisp.