The pinnacle of video game storytelling.
The worldbuilding is genuinely phenomenal, most of this is due to the writing, it's great. It can be humorous, witty, introspective, or just about any other good-sounding adjective you can think of, and it's just about always compelling. The gameplay may leave a bit to be desired, but the universe is so fully realized and intertwined in ways that captivated me from start to finish.
I agree the ending had more potential than what we got, but I think it's a shame that ME3 is only remembered by its ending than by all its beautiful moments.
If only I could experience this trilogy for the first time again.