I preferred the AI in Halo over Half Life, but Half life was probably more revolutionary for it's time.
Of course, there's so much more to AI design than just the AI routines themselves, for example the sound design in Half Life makes you totally aware of the AI's Mindset. Marines announce their every little move, and then you think they are smarter than they actually are.
"I'm Flanking!"
"Throwing Grenade!"
Another example of this is encounter design. For example, FEAR has great enemy AI, they are smart, take cover, and flank really well. Unfortunately, 90% of the encounters you face in the game are shootouts down long corridors where that AI never gets a chance to use anything but the "Duck in and out of cover" abilities.
I think Halo's real genius is how it uses an AI that appears smart, doesn't cheat, and uses it to force the player to plan and adapt. For example, while picking off Grunts is good strategy in a conflict, if you can quickly bring down an elite, you can scatter the lesser units. But then you miss, and you now know you need to re-adapt your strategy, because the Elite is in cover, and the grunts are laying down suppressing fire, with some jackals preventing you from getting an angle on that Elite in Cover.