I think Half-Life 2's decision to avoid omg teh badazz guns makes the game more realistic and more practical. It's not about hardcore gunplay; it's still there, but what impresses me the most is the story, the detail in everything (characters, character behavior, environments, etc.), most of the "puzzles" feel so real and connected to the world... it's just a very well-executed and impressive franchise. After seeing Halo 3 through to the end, it has absolutely nothing, zero, zilch, on Half-Life 2, Episode One, and Episode Two. It's a shame that Episode Two is taking a backseat to the Halo 3 hype, because it's bar none a superior FPS. Yes Halo 3 has the multiplayer aspect and that's dandy and all, but I consider Halo 1 and Halo 2 to have great single-player experiences in addition to the multiplayer, and I found Halo 3's single-player to be extremely shallow and relatively non-eventful.
I was kinda bummed that I didn't have a 360 when Halo 3 launched, I hate missing out on the party, but after buying the Orange Box and seeing Halo 3's story mode, I can't say I'm feeling the pain anymore.