What is the easiest to operate/fastest solo bitcoin mining client for a cluster rig? I'm planning to set up a rig and need a solo mining client. Can you offer me some suggestions? (Please don't just tell me that solo mining isn't worth it).
If you have a GPU rig cluster, you ought to look at standard miners. This question should help you get started:

What open source miner applications are there?

In the end you ought to try a few of them out and see which ones give you the best performance on your hardware. You can also have a look at Mining hardware comparison Wiki page to see what performance you should expect from your hardware and what configuration was used to accomplish it.