Thats a toughy, but when I think about kits I truly enjoyed building begining to end and couldn't complain about anything in them, my vote would have to be Italeri's 1/72 T-45 Goshawk. Well thought out and a breeze to put together. I think its one of the only kits of the American version of the Hawk going outside of a 1/48 Collectaire multimedia kit.
My T-45 met with an unfortunate end during a move and I've been waiting for it to be re issued ever since. Perhaps with the soon upon us retirement of the T-2 Buckeye, that T-45 will come back to the shelves.