I haven't played CoO, but I'm gonna have to go with GoW2. The story got a little wackier than I had hoped towards the end, but the gameplay was much better than the first game. When I played through GoW and GoW2 on the PS2 some time ago, I felt GoW2 was a bit tedious after GoW, but now that I played through them in the Collection, it was the opposite. GoW was a bit 'meh', but GoW2 rocked harder than most games do these days!
And yes, cliffhanger (meh), though it is possibly the most awesome potential a game has ever given its inevitable sequel 