Meh, everyone knew Rogue Agent was gonna be crap though.........
I'll have to say Halo 2 Single Player. I feel sorry for XBox Live-less people, because the story is awful and nowhere near as fun as the original. It also seemed to be too much like the last one (nice start, dreary-boring-alien-ship-middle, and an even crappier ending).
Halo 2 is great online, but man when I fired up single player I was expecting a whole lot more. It was really average - especially when we had Half-Life 2/Counterstrike out at the same time.
Halo is good and all, but I thought I was trippin' when I played it - surely I was missing something and somehow it's better than the mundaneness I was experiencing.
I even bought Halo 2 before Half-Life and could still see the flaws, and I only dread to think of how crappy it would have looked had I played it AFTER Half-Life 2.