So what I'm seeing here is:
1)ME1's ending is every bit as badly written and full of holes as ME3's ending yet it gets a pass because the player felt "heroic" and got to crawl out of the rubble at the end. Hence it is a great ending.
2)ME2's ending was badly written because it had the terminator and allowed critical characters to die setting up numerous problems for writing a better ME3 plot, but it gets a pass even though something magical appeared in the floor of a floating platform that caused the entire station to either detonate or have a timed radiation pulse. You survived. You felt heroic. Hence it is a great ending.
3)ME3's ending sucked because nothing masked the crappy choices and plot holes. You died, the relays exploded, and the Normandy crashed. The only real difference was the color of the explosions on your screen. It was art. Dragon Age Spoiler -
The player didn't feel heroic. Hence it sucked.