GoW 1 kinda sucked. It had great graphics at the time and some boobies so it was okay. I don't use this word often but it really was overrated. So much so that I didn't even play GoW2 until collection came out.
GoW2 was great. At the time, some where saying it was a step down. I don't know what they were talking about. I thought it was great. Couldn't stop playing it.
GoW3 was pretty awesome. Not for the fighting (I find the combat in all 3 games to be a little lacking--looks cool, though) but the scale, story, and overall visceral feel of it made it stand out.
I say force your way through the mediocre GoW1 and enjoy the next two. I also got the PSP games and, while they were great for portable games, they weren't really what I'd call great games. Ghost of Sparta was waaaay better than Chains of Olympus, though. Both were fun diversions.
