You can plump for Twitterrific, TweetDeck, TwitBird, Tweetie, Twittelator, Echofon (which rather surprisingly doesn't begin with T but was previously called TwitterFon, so that's OK) and a whole host of others which don't appear on the first couple of pages of search results.
But which one?
Reviews abound on the interweb declaring their undying love for each, and slagging off the competition. Perhaps most people just get used to the first one they try and can't be bothered to change. But that's boring, so I'm going to try them all.
For a period of two weeks, every time I go to check Twitter on the iPhone, I'm going to use a different one of the above six apps, in strict rotation so they get a similar amount of face time. To be fair, I have installed the free versions of each, so no paid-for functionality can muddy the waters. The settings will be identical and they all support the same photo-sharing and link-shortening services. At the end of the fortnight, I should know which ones I've looked forward to using and been most 'tweetful' (ahem) and to which I've given a little sigh and been as quick as I could.
Maybe there are magic features you can't live without, or perhaps it's all about speed of use and a clean interface, but at the very least I'll have my Twitter app (which I'm sure everyone will then tell me is "the wrong one"...)