Smartest, most efficient and logical AI isn't always the one that offers the most fun game play experience. Sure, it should offer a hard enough challenge, but I feel that a "good AI" requires a lot more than an opponent that does all the right decisions in every combat scenario. AI should have varied, fun and unexpected reactions to players actions so that a player could never be 100% sure what an opponent does, even if the "right action" was obvious. AI can and should do wrong moves every now and then. Stupid mistakes are fun, as long as there's some logic why the enemy does what it does. Moving back and forth between two covers burning all the TU's and not run away or shoot a Xenonaut standing in the adjacent tile is wrong kind of stupid behavior. Don't want that.