What's the most improved sequel in your opinion.

I think AC2, I enjoyed it alot more than the first one in every way. The thing is though it had no replay value, so It's a good rental.

Assassin's Creed II for sure.
I thought the first one was pretty crappy. Then the second one comes along and blew my mind on how fun it was.

Bad Company 2. The first one was crap but the second one was much better though i never really liked either game much.

Assassin's Creed II because the first one was mediocre

Assassins Creed 2 for me. I really enjoyed the first Uncharted so while the second one was an improvement it wasn't as big of a leap as AC1 to AC2. I can't wait for AC3 if it continues this trend it will be mind blowing!

Easily No More Heroes 2.

The first was genius, the second was a masterpiece.

AC2 by a long shot.

Diablo 2. Out of that list i would say KZ2.

I think, from all of those game, Red Steel 2 is the one.

Could be Killzone 2, but you can't really compare it with a game from the last gen. And, i haven't played any Mass Effect.