Actually, of all things, Warcraft 1. The computer started making feints and flanking maneuver after a time, and seemed to grow and improve as time went on. It also actually probed for my position and strength in a location, and altered its plan based on that, instead of just continually throwing wave attacks at things. Frankly, the AI in Warcraft 2 and 3 never seemed to reach the same level as the original. Still not as clever as a human opponent certainly, but...instead of playing the game of 'the computer is a cheating bastard' or throw things straight at you...it adapted and developed a reasonable strategy.
Sadly, it never learned how to deal with certain things, like the summons of your mage's...you could use those to play wild hob with its plans as decoys and the like.