There’re many versions of GNU Emacs to choose from when it comes to MacOS. There’s a standard console-only Emacs that comes with the system. There’s Carbon Emacs that (as the name implies) uses Carbon libs. There’s an Aquamacs — again, the name is telling. But what I found to be a very nice addition to this group is Emacs.app — a Cocoa-based Emacs version.
It is still in development and shows a few rough edges here and there — but I find it a lot more responsive (and a lot less pretentious) than Aquamacs, while being a proper Cocoa application (as much as it can be said about Emacs). Moreover, its code base is a lot more current than either of the alternatives, hence you get a few added whistles, like code checking in most popular languages (e.g. Perl).
To be fair — there more Emacsen to choose from, see this page on EmacsWiki.