I really enjoyed Dragon Age 2, Mass Effect 3 and Fable 3... So my opinions here will probably count for shit, but I've seen Destroy All Humans 2 mentioned and I'll have to go with that. The first one was so much fun, I can't even describe the fun. But the second one sucked, especially when it came to the disguises. The comedy wasn't so great, either.
I found Splinter Cell Conviction to be a let down, too. Not just due to gameplay but Ironside's voice wasn't up to scratch, either. I enjoyed the gameplay, up to a point.