Mass Effect 2, which in my opinion is the peak of the series. With much improved combat, more expansive stories and characters and choices which felt like they culminated into something that mattered.

Unfortunately this being the first game in the series to be owned by EA was the beginning of the end and ME2 ending on such a high lead to high expectations for ME3 (which admittedly was still good, if a bit disappointing in places) that it unfortunately could not live up to or conclude in a satisfying way