I was just discussing this with another member so I'll throw out my 2 cents. The Tang Band titanium 4" and bamboo 4" both sound great, and really deliver once DSP is applied. By far the best wideband driver I've ever heard is the 2" & 4.5" Jordan variety. Incredible revealing drivers. They're in the camp of bending wave drivers that behave in a way that causes the apparent diameter of the driver to decrease as frequency increases, which allows for incredible off axis performance. They're spendy but well worth it. A couple other flat cone, bending wave drivers can be found on
Parts Express for next to no money and those also sound very good. I forget the model/part number at the moment. HiWave or something like that.