MacDonald IMO has all the natural skills to beat Diaz. Its just a matter of application.
As far as I have seen from watching both of the Diaz brothers, their style of boxing rely's very heavily on the ranging jab. When you take away the usual reach advantage they have against their opponent's. I.e. Dan Hardy (74in) vs Diaz (74in) a lot of the usual advantage goes away. And in my opinion, Hardy is a good combination of pressure fighter and technical fighter. (Also, he has a Shaolin Kung Fu background which gives him an intangible leg up for me!
Kampmann has very technical with his striking, and has the grappling skills to keep Diaz standing. I think a skilled technical kickboxer would give Diaz a lot of trouble that someone like Noons, who's style is almost exclusively boxing w/ leg kicks doesn't have.
Whether they'd fight or not has absolutely nothing to do with their rankings. If they did, Shields would secure the takedown and dominate with far superior wrestling.
I totally agree with Condit being a dark horse. He's so underrated it blows my mind (which is why he's higher up on my list than Hardy). I have him beating Hardy, and continue to kick myself for only putting 20K on him to win.
Actual bottom line: Diaz being even close to top 5 is very debateable, and something of a stretch.