F.E.A.R. 
Half-Life 2 Overall Comparison The enemies' AI is what put F.E.A.R. ahead in this comparison. While fights can be pretty big and intense in HL2, nothing compares to trying to outthink the clones in FEAR. Nothing is more exhilerating than listening to your enemies radio chatter as they plot out a plan to close in on you. Add that to the fact that the enemy's weapons hurt you just as much as you hurt them, and you got yourself a winner.
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Comments
Both these games scared the pants off me :(
This is a good comparison, and I have to agree that the AI is better for fear, but we can't leave out the story, I the story for HL2 was just absolutely amazing. Not to mention Source > Havok.
The story is better in HL2, true, but I love the atmosphere and the horror so much more. Both are told completely through a first-person perspective of no one but the protagonist, but halucinations are more interesting IMO.
Also, Source is a derivative of Havok, hence the Havok logo for HL2.