The best AI isn't the same thing as "hardest to program," "most complex," or "most difficult to fight."
The best AI is the one that makes you briefly feel like you're possibly interacting with something beyond a computer subroutine, something with some personality.
To have an enemy that stands in such stark contrast to the rest of the game, one that just stares you down, you draw your weapon and it does the same, sometimes firing at you behind cover or charging at you...it was designed in a really smart way. So many people in this thread are talking about them in ways reserved for real people or characters. They seem cautious, curious, alert, intimidating. Intelligent.