I have only tried a couple so far, but I can say without a doubt that TwitterFon is going to be hard to beat for me.
It is not the most robust app you can get, however, it gets the job done in a very concise manner and that is incredibly important to me.
To the right is an actual screen capture from my iPhone to show the clean interface of TwitterFon.
One of my favorite aspects is the Search and the ability to keep previous Searches that require just one touch to get reloaded. If you mis type a search or do not want to keep it, you can simply remove it from the list.