FeFederer's tweener against Djokovic USO SF '09.
Easily the best.
There may have been "harder" ones maybe, like Federer's own repeat in 2010 again Dabul. But its hard to top a cross court tweener on a HC in a Slam SF to bring up match point.
By the way, you must include the reverse hot=dog of Federer's against Schoorel in Doha this year. Federer managed to hide his dorkiness of outrunning the ball by hitting that tweener out of the blue.