Have You Played? is an endless stream of game retrospectives. One a day, every day of the year, perhaps for all time.
I know you're supposed to say the first one's best, but Mass Effect 2 is definitely my favourite in the series.
For many, ME2 is a harbinger of doom, marking the point where Bioware most dramatically shifted from making what we deemed to be RPGs to the combat'n'conversation structure that, to a greater or lesser extent, characterises the studio's output now. ME2 is far more action heavy, and setpiece-heavy too, moving even further away from stats towards pick'n'mixing destructive powers and talking-based dilemmas that were more about an on-the-spot moral choice than using guile and skillpoints to tease your way to the optimum result.
And then, in its final act, everything culminated in a glorious final stand movie whose potentially lethal consequences might have felt a little arbitrary, but were closely related to how well you'd treated your memorable gaggle of companions throughout. Few big-budget games have managed to feel truly climactic in this way. Mass Effect 2 has a fair bit of series-typical cruft, but at heart it's the Dirty Dozen in space, and I love it for that.
It also introduced an air of sinister mystique that I hadn't felt the first game quite achieved, in your new boss thee Illusive Man and his unclear intent for your team and, indeed, the galaxy.
Mass Effect 2 is the point that the Mass Effect series truly became the Mass Effect series. I would agree that it's been diminishing returns to a greater or lesser degree since, but for me that absolutely makes ME2 the high point, not the point at which it all went wrong.