Which Twitter App?

Tweetie is a full-featured Twitter client for the iPhone with an extemely fast and responsive interface. This twitter app tailors to the power users while being easy to use for beginners. Features include:

• Well-done UI: Tweetie has exemplary user interface design as both a Twitter app and an iPhone app. Very fast, responsive and well designed, with most features within a couple taps. Settings are stored within the global Settings on the iPhone.
• Helper Apps: Choose between TwitPic, yFrog, and Mobypicture for picture uploading. A web browser is built in, with the ability to tweet or email links you want to share.
• Multiple Accounts: track multiple twitter accounts. No Facebook integration as of version 1.2
• Searching: save your searches with one button press. Check Twitter Trends and even customize them before saving.
• Converations: Pull up replies and conversations using the advanced search. Follow conversation threads when they are available ("in reply to..." & "view reply").
• Drafts: save a tweet for later, or if you get interrupted by a call.

Well Worth It

While it's not free, Tweetie is well worth the small amount the developer asks for. It packs all the functionality of a full-featured Twitter client in a small, well-designed interface, and is a favorite amongst all the apps reviewed on this site. @atebits, the maker of Tweetie, created a companion on the Mac desktop, Tweetie for Mac.

Updated Version

UPDATE: You can now get Tweetie 2, which is amazingly powerful while retaining all of its simplicity. It was completely rewritten and has slick, trend-setting iPhone action, such as Pull to Refresh and powerful #hashtag and @username pickers, among dozens of other improvements. Read our Tweetie 2 review.