Eh? How has nobody recommended Twittelator Pro? I've used it on both the iPhone and iPad for over a year, and (despite trying out about 10 others), have never come across anything even remotely close to it.
For example, as far as I'm aware, it's the only client that shows image previews in the tweet timeline. Considering how important posted pictures are to Twitter, I simply cannot accept this as anything less than a core feature. It also has translation options, works well with lists, has fully customisable themes and accurately displays (and remembers) the number of remaining unread tweets.
How can I be one of the few who appreciates these features? Maybe... maybe you guys just don't know of their existence, and therefore don't regard them as essential.
Anyway, check it out. There's also a free version, but I can't vouch for that. It's probably great.