The official Twitter app for is capable of handling multiple Twitter accounts and features an ncredibly seful offline mode with the added option to save to Instapaper for those occasions when you do not have connection available.
The app includes live filtering that highlights particular types of tweets. Your own tweets can include attachments such as videos and images, and file hosts can be easily configured from within the app.
One other handy feature is that should you lose your connection in the middle of writing or creating a tweet and Twitter goes offline while you're doing so, a draft manager is on hand to ensure that your typing is saved.
The latest version has received an improved design (although not on the iPad, strangely).