If you ever plan on making a serious effort at learning Emacs, immediately forget about Aquamacs. It tries to twist and bend Emacs into something it's not (a super-native OS X app). That might sound well and all, but once you realize that it completely breaks nearly every standard keybinding and behavior of Emacs, you begin to wonder why you aren't just using TextEdit or TextMate.
Carbon Emacs is a good Emacs application for OS X. It is as close as you'll get to GNU Emacs without compiling for yourself. It fits in well enough with the operating system, but at the same time, is the wonderful Emacs we all know and love. Currently it requires Leopard with the latest release, but most people have upgraded by now anyway. You can fetch it here.
Alternatively, if you want to use Vim on OS X, I've heard good things about MacVim.
Beyond those, there are the obvious TextEdit, TextMate, etc line of editors. They work for some people, but most "advanced" users I know (myself included) hate touching them with anything shorter than a 15ft pole.