I personally like PlainText more than any of the other text editors I've used. It's clean, easy to use, allows you to delete files and folders with a swipe, and Grosjean has obviously put a lot of thought into the design of the UI. The app icon is a large, black editor's paragraph symbol on a white background, which is extremely easy to find. The app is free, using iAds to defray the cost. Grosjean wants you to be able to remove the ads by doing an in-app purchase for $4.99, but the in-app purchase option is still in review by Apple.
Next, there's Elements (US$4.99) from Second Gear. This is the app I've used for quite a while, until one of the other bloggers here at TUAW pointed out PlainText this morning. As with PlainText, Elements is a plain text editor. In the file list mode, the newly-updated Elements lets you sort documents by name or modification date. It does not, however, let you create folders in Dropbox, which I find to be a real negative.
Elements has support for spell check and TextExpander, and it's possible to change the font, text size, text color, and background color easily in order to make it easy to read the document. Do you want green text on a black background? You've got it.
I know this is a nit-picking comment, but I don't like the icon for Elements. It's supposed to be a composition notebook with a black-and-white speckled cover, but to me, it looks like static on an old TV instead. Elements is a universal app, running on any iOS device.
Droptext ($0.99) from Invisions Technical Arts is another universal app that works well with Dropbox. It's the least expensive of the apps (if you don't count the ad-supported free version of PlainText), but it has many of the features that the other apps enjoy -- it has TextExpander support, it can create files and folders from within the app, and it allows customization of the writing environment (font, size, text and background color).
iA Writer ($4.99) is an iPad-only app, which makes it slightly less useful than the other three apps we've discussed here. From Information Architects, Writer is probably the most focused on just ... writing. It has a Focus Mode that takes all distractions off of the screen. There's no auto-correction, no spell check, no toolbars, nothing. You can clearly see the last three lines of text that you were working on; everything else is grayed out.
The app has a beautiful monospaced font that it uses to make your text more legible as you're reading over it. There's a tool that calculates how long it will take to read your text; that's great if you're writing a script or speech. Information Architects is planning on adding auto-syncing, subfolders, word count, and other sync services in the near future.
If you know of any other iOS text editors that work with Dropbox, let us and other readers know by leaving a comment below.