Well I've been using Tweetdeck for a while but they've had problems and don't seem to care to respond to user queries.
So I started looking around and found Mixero - also still in beta and I love it.
It does most of what other clients do but adds in translation (be nice for it to be auto); shows a converstaion; allows to create a group for a keyword but block specific users; does multiple accounts; context (create different profiles for where you are - so could work in a ministry environment); gives a preview of pics and shortened URLs; allows for contact grouping (including grouping across accounts); allows for your channels or groups to be displayed across multiple windows and much more.
Of course, being based on Adobe AIR its universal as a client and the major plus for me is that the developers respond to you.
You'll need to request a code to use it, but the developers responded to my request in udner 24hrs and they have an iPhone client on its way.
I likes it. I likes it a lot
http://www.mixero.com/