I vote against Furmark. It's an exceptional stress test, but it's perhaps too good. It stresses the GPU far beyond any measure of practical performance. Cards that are 100% stable in real world use will regularly fail Furmark stability tests because it's just too hard on the card.
Some have even said that it can cause damage through overheating and should be avoided for that reason, but I'm not sure I buy that. Personally, I use ATI Tool or the
[email protected] GPU client. If it can run either program for a long period of time, then I consider the GPU overclock stable.