Assassin's Creed (the first one) was really good. Sure, the investigations got repetitive, but they were pretty much just filler between some of the really awesome assassination missions (the whole de Sable part, from the city to the battlefield, is up there with one of my fondest memories of any game). Combat was easy but still very satisfying due to how brutal it looked and how simply you could do stuff that looked cool.
Assassin's Creed 2, on the other hand, went and fixed some of the flaws from the first game. They took out the investigation missions, made scaling buildings a less laborious process, let you run past a guard without them chasing you ("Only an assassin would run! My logic is infallible!"), and added some more fun challenges. The only problem I had, in retrospect, was that some of the new additions seemed to be "just because". After the introductory training session, I didn't use the poisoned blade once. I didn't know you could throw sand into a guard's face during a fight until I saw there was an achievement for it. Stuff like that. Although I suppose if that's the only criticism I can level at a game ("There's too much stuff!") then it can't have done too wrong by me.