Have to say Glentress. Kirroughtree has the (far) better individual pieces of riding but GT Black is a better trail as a whole. In the future, when people take BScs in trail building, GT black will be case study #1. Japanese exchange students will line the side of the trail, trying to understand the inner narrative of Deliverance (if they don't do so already). Kirro will just be an example of some ace rocky bits that got joined up with some paths.
I've also only ridden Kirro twice, and loved it, but ridden GT 100 times, and still love it, so I reckon that must count for something.