Coolest enemy feature in Halo

So far from what I have played in Halo, there is one thing the enemy does that I think is way cool. When you are trying to fight your way through the ship to escape, the aliens can throw a shield out ahead of them. One time, I was charging right at a group of them guns blazing, and they threw it out way ahead of them right in front of me, and I smacked right into it, it was so cool. The enemies shields of all types is very very well done. and I do agree, the enemy AI is very good as well. They really move and dodge and hide in reaction to what you do, its great AI.

As if we needed another reason to buy Halo...