I own Halo on xbox, pc, and i have the LE edition of Halo 2. I still think that Half-life 2 >> Halo 2. It's no match. But for some reason, Both games will take all my free time for weeks to come.
I'm weird, just like that.
Also; I have no idea why people say that Halo's AI is special. I finished halo in every damn level of difficulty, several times, and I never noticed something different. If for some reason that Elites jumping on top of boxes and spotting you and having them dodge your grenades and hide behind a box is "special", homie please, go play more fps games before spewing bullshit.
The things that make Halo fun is the physics engine, overall presentation and incredibly fun multiplayer modes. But stop talking about the A.I in Halo being some sort of hallmark in modern computing. The real challenge in Legendary mode is that the damage is higher. That is all. I never noticed anything special about their tactics, except maybe for some reaction time, in every playtrought.
I never thought "Oh my, I cant believe the a.i did that" even once since I started playing the Halo games.