there are quite a few “integrated” solutions, combining editor and pdf preview (the very fine TeXShop being one of them) -- some of them are quite new (even beta); TeXShop, on the other hand, is very mature:
Texmaker (cross-platform, qt-based; all the others are OS X native):
You can also check the entries, and the comments, on Macupdate:
That said, I use none of them, although I have TeXShop around. (This doesn’t mean the new integrated solutions aren’t any good; I haven’t tried them yet.)
That’s because what’s most important, imho, in a good LaTeX environment is the editor, i.e., how much help is there to get the source written. Accordingly, many people would agree that the best editor available plus a fine synchronised, but stand-alone pdf viewer is superior than a (compromising, or so the musing goes) all-in-one tool.
The, hands-down, best pdf viewer, supporting PDFSync (syncing source and pdf), is Skim; it’s free:
As regards the best editor, there is constant disagreement among OS X users, some would even say “war” -- but that just shows how vivid the Mac ecosphere is when it comes to such tools.
Leaving the “classics” aside, Emacs (Aquamacs, http://aquamacs.org/) and Vim (Macvim, http://code.google.com/p/macvim/), dreaded for their learning curve, worshipped for their power, two of the best, modern, clean, yet powerful editors are TextMate and Sublime Text 2:
Many would hold that the combination of either TextMate (2) or Sublime Text 2 together with Skim provides the best LaTeX environment ever.
I was looking at this thread and it's been a while since it started, but I hope the answer can still be useful:
If you want to be able to have a good visual interface while creating your content, LyX is a very good option. It is a word processor that uses LaTex as its engine. It's the one I like to use the most because it can be very effective even if the user is not really familiar with LaTex code.
I started using Texpad, which is available from the Apple Store and does quite a good job. It is commercial, but the result is the most convincing that I have seen for quite some time. The way how the result/errors are presented is what I am looking for. And it shows the complete input tree.
I wish it would show the different LaTeX commands too. But it comes with autocomplete. So, if you know what you are doing it is supporting you.
For lighter-weight mathematical writing, check out Archimedes: http://www.mattrajca.com/archimedes/
Archimedes combines LaTeX and Markdown in a native OS X app and doesn't require external TeX packages to be installed. It features Xcode-style command completion, live previews, a math library, integration with OS X's share sheets, and more! You can even insert mathematical symbols by drawing them on your MacBook's trackpad or Magic Trackpad.
Disclaimer: I am the developer of Archimedes.