The question is basically in the title
I have some clients interested in development of their own ad serving platform and I was wondering if you could advise the best UX that you've come across over the years of your professional career as AM?
Go2mobi
I like a lot of features on decisive, but the actual programming behind it is absolute shit.
Zeropark
Zeropark looks nice and easy, but is for advertisers only? Haven't found a publishers section yet.
Zeropark, really, with that damn iFrame?
Google AdWords
Tapit is pretty slick, decisive would be my second.
While waiting for some approvals, I've come up with sitescout.com which looks very intuitive.

This site scout looks like stock exchange for ad networks - how do they pull it off?
In terms of pure interface, without considering the speed issues, Decisive would win it for me.