8/10, but I've only played singleplayer and have no greater interest in multiplayer (and I can't even play multiplayer anyway).
Halo 1 was a fun ride. The story was interesting enough to hold my attention, and had this kind of mysterious touch, it made me feel like I had no idea what's would come, exploring the unknown etc. It made me want to know more about what's gonna happen. The setting, weapons and enemies felt refreshing. The system with only being able to carry two weapons was cool, and driving vehicles and playing in co-op.
Halo 2 was fun too, but...it doesn't have the same impact on me as the first had. The story is also overdone with too many cutscenes, sometimes dragging on like a sub-par Star Trek episode. It actually made me lose interest in it, and I ended up not paying much attention to what I actually was supposed to do. Halo 2 still also has the boring level design which caused me to get lost a few times. And it's not exactly exciting to look at seemingly endless sterile corridors covered in 75 % darkness. I much prefer the outdoors forest/jungle settings.
Playing around with vehicles is still great fun though, and so is co-op.
But the game itself has a formula that's not as fresh as it was back when Halo 1 came out, and it also has a more confusing and unfocused story (the ending didn't help either). I was expecting more with the three years of development in mind. I was also expecting more because Halo 1 was kind of rushed for the Xbox launch, but with Halo 2 they had a lot of time to get things right.