Tweetie 5

Hands down the best twitter client available for the iPhone. What makes this app so great is that it is fast, full featured, and has a highly polished interface that makes it feel like it should have come with the iPhone. Scrolling with many other twitter clients is slow and glitchy, but Tweetie handles it smoothly. It also has a bunch of power user features like trends, saved searches, multiple accounts, posting photos, and more.