Well have to say it is to this day still ME3, and I nailed that in first playthrough without the internet, any walkthroughs or anything, with best possible ending. Pre-extended cut. (Although this gripe still applies post-extended cut).
The choices to this day still feels very unpersonal and not applicable to the universe as much as to general philosophical and ethnical questions.
That and I had a feeling I was playing Deus Ex: Human Revolution, but without any real context/connection to the ending choices. As these things didn't really take into account Shepard's journey and experiences as much as maybe Jensen's dillemas,
Existence and life are two entirely different concepts to revolve choices about. When you think about it.
However that one was obvious so...
Another one would have to Legacy of Kain: Defiance. This one on the other hand is more of an immersive sense of "least satisfying victory", it ends on a very bittersweet note. So I suppose that is at the opposite end of the spectrum. It barely even felt like a victory, more like a turning point - which in this instance is a good kind of thing.
Although it does have the downside that it never really concluded the series after that cliffhanger. Still, good a series, good games.