I think I'd say Halo: Reach. Yes I know it's a prequel but it's a sequel in all but name to the instalment it followed (Halo 3), but I'm talking more about it's expansion Halo 3: ODST. ODST had a lot going for it with an interesting plot to say the least, tried changing the formula (even if it only did it slightly) and even had it's own mini-campaign within it which you discovered by collecting audio logs. It was also atmospheric and it didn't tend to bore me.
Halo: Reach tried to do the same things as ODST in it's own way and massively failed on them. Characters were boring, the gameplay additions proved arbitrary or just plain idiotic to consider using, the Firefight system was smashed into bits, story was cliché at best and the ending lead to useless speculation. Also, following the trend of most 2010 games I've played, I haven't bothered replaying it because I lack the motivation. It was massively disappointing with it's only two positive moments are when it seems like the better Halos that had come before and despite it's flaws felt like a worthy "ending" the series Bungie had worked so hard on.
Halo: Reach tried to do the same things as ODST in it's own way and massively failed on them. Characters were boring, the gameplay additions proved arbitrary or just plain idiotic to consider using, the Firefight system was smashed into bits, story was cliché at best and the ending lead to useless speculation. Also, following the trend of most 2010 games I've played, I haven't bothered replaying it because I lack the motivation. It was massively disappointing with it's only two positive moments are when it seems like the better Halos that had come before and despite it's flaws felt like a worthy "ending" the series Bungie had worked so hard on.