Levels/Puzzles/Bosses/Story in GoW2 >>>>>>>>>>>>>>> GoW1
The only thing I will give GoW1 is that the enemies are more vicous and so the combat is a better challenge. GoW1 has awful puzzles. Most involving box pushing. GoW1 has boring locations, desert yawn, pandora temple, meh. GoW1 has awful boss fights. Hydra sucks, Ares sucks. GoW2 has some amazing boss fights, the best stage design and locations of last generation (climbing the colossus, the horse statutes, flying with pegasus, etc...). GoW1 was also about a dumb guy named Kratos and his personal story that you really can't care about because he is so dumb. GoW2 was all about Greek mythology, going to fantastic places and fighting ancient heroes.
GoW3 has level design somewhere between the two, some great locations but not as many or as varied as 2, puzzles about on par with GoW2 but not as many puzzles as GoW2, combat is the best of the series, boss fights are better than 1 but worse than 2, and story is better than 1 but worse than 2.