Dragon Age 2. Fight me, haters.
X-2. Embrace the girl-power campness.
XIII-2. I like it, so there.
Borderlands 2. The first game felt like more of a proof-of-concept, while 2 was a fully-fleshed finished product.
Fable 2. The first Fable is overrated, and I really feel that 2 improved upon it in pretty much every conceivable way.
Uncharted 2. Don't get me wrong, I liked the first one. But the second one blew it out of the water.
Mass Effect 2. Despite a thin plot, it was a much slicker, more compelling package than the original.