I finished Mass Effect 3.
Shortly after finishing it, at about 1 AM Saturday morning, I tweeted that I was overwhelmed. The game's final hour is emotionally exhausting; while ME2 has you confronting a suicide mission with your squad, in 3 you are facing the extinction of your own species and every other sapient race in the galaxy at the hands of an enemy as horrifying as it was intractable and unknowable, and the chances of any kind of success, let alone your own personal survival, seem increasingly remote.
The final hour of the game is the grim culmination of a game shot through with a sense of dire urgency. It's a triumph of quest design, in this regard, because practically speaking it's no different than any other RPG. It consists of a handful of lynchpin plot-advancing missions between whose interstices you're given more or less free reign to wander around and do sidequests. But because of the particular nature of the sidequests, the technical execution of the environments you play through, and the overall thrust of the narrative, it always feels like you have too many urgent things to do, too many top-priority, every-second-I-put-this-off-more-people-
By the end of it I was exhausted. My lady, Cmdr. Sora Shepard, was a round-faced, multi-racial, idealistic engine of relentless psionic destruction, but after 35 hours of bargaining with the suspicious, cajoling the reluctant, threatening the traitorous, and encouraging the weary, she was tired too.
And as you approach the final conflict, the tenor of the conversations you have changes. Everybody knows what's coming. Everybody hopes you'll be successful, but nobody is pretending to think you'll live to tell the tale.
At that point, just before the final assault, the game had been as close to perfect as anyone could reasonably expect. Shepard helped Mordin cure the Krogan Genophage and redeem himself in the process, and she helped Wrex deliver that cure to his people and fight a battle his great-grandchildren will tell stories about. She made friends with a weary Krogan shaman lady whose suffering purchased hope anew for her entire species. She met up with Tali, whose people were embroiled in the ill-advised climax to a 300-year-long war that was their fault, and overcoming rancor and mistrust on both sides, brokered a reconciliation that returned the Quarians to their homeworld without destroying the Geth, a reconciliation that most galactic citizens would've laughed at as flatly impossible.
And all these things and the scores of tiny beautiful moments in between came about as a result of the choices I made and the individual relationships I cultivated. 35 hours into Mass Effect 3, the thesis of the game and the trilogy as a whole was that even on the galactic scale, individuals and their choices matter―and this thesis was stronger than ever.
Shepard fought her way up to the Citadel, confronted The Illusive Man, and convinced him―as she convinced Saren before him―of the error of his ways, whereupon he chose to take his own life.
As she lay there, bloodied and presumably dying, she had an enormously affecting exchange with Anderson. Can't really think about it, honestly. Keith David, ladies and gentlemen. But then the the war-ending macguffin did not activate. Instead, a levitating platform carried her body to the top of the Citadel, where she confronted a spectral version of the little boy who'd literally haunted her dreams ever since she saw him killed by a reaper in the game's opening scenes.
The boy explained that he was the master of the Citadel and the reapers , and that the reaper-enforced extinction cycle was his way of avoiding the conflict that he claimed was inevitable upon organic life creating synthetic intelligence. Organic intelligence would always eventually create synthetic intelligence, and would always be extinguished by it.
But ours was the first cycle to complete the Crucible , and Shepard was the first organic being to talk directly to him. Her reward for this was being allowed to choose one of three options.
Use the Crucible to destroy the reapers. This would also destroy all other synthetic intelligence in the galaxy, namely, my Geth allies and EDI, my ship's AI. 
Use the Crucible to control the reapers, force them to retreat back into dark space, and be consumed in the process. This is what the Illusive Man tried and failed to do.
Use the Crucible to synthesize organic and synthetic life galaxy-wide into a new, hybrid form of life, and be consumed in the process.
The first option was colored blue in the ending, the second, red. Those correspond to the game's chromatic language for "Paragon" and "Renegade" morality choices, but make no sense in light of the general nature of those choices in the story so far. The "Destroy" option entails genocide of a race I liberated mere hours earlier. The "Control" option is untenable for the same reason it's always been untenable―every attempt to control or interface with the reapers has ended with the corruption of the would-be controller, and save metatextual "I'm at the end of the game"-type reasoning, I had no reason to think this attempt would end differently.
Oh, one more thing, the boy informed me: No matter what option you choose, the mass relays will be destroyed in the process. 
Which left me at Synthesis, which I chose. Shepard was consumed by a fulminating column of green light, which propagated through the Mass Relay system, destroying reapers and relays as it goes. Cut to a scene of Joker piloting the Normandy between two relays . He fails to evade the light, which strikes the ship, damaging it badly. It crash-lands on a jungle-covered planet.
We pan across the garden world's greenery, which is shot through with circuitry. Apparently I have successfully merged organic and synthetic life!  Joker and EDI get out of the ship and gaze Adam-and-Eveishly across the landscape. Liara emerges next .
Then there's a scene of an old man and a boy looking up at a starry sky, and the old man tells the boy that's the end of the story of the Shepard, and that there might be other life around all those stars in the sky, isn't that crazy.
So I blinked and went to bed, too overwhelmed with that loose-ends feeling that comes when you finish any narrative you've been deeply invested in to really process the particulars.
The more I thought about it, the less happy I was. I awoke the next morning and gorged myself on ending reactions from across the internet, only to find that no matter which of the three given options you pick, the ending is largely the same, save for the color of the relay-destroying column of light. 
Perhaps you have noticed the bracketed numbers. Let us now confront them.
 If this godlike AI construct is indeed the master of the Citadel and creator of the reapers, why in the name of Odin's one eye did any of Mass Effect 1 even happen? The whole point of that entire game was stopping Sovereign from getting to the Citadel and manually activating its relay, but if the Citadel has its own intelligence, which exceeds that even of the unknowably vast godlike reapers, why couldn't it just activate itself?!
 At this point I wanted to grab the kid by his shimmering spectral shoulders and yell "Look out the window, asshole, I ended a war between organics and synthetics and convinced the synthetics to abandon the reapers and fight at my side!" The fact that I couldn't cite my hard-won solution to the Quarian/Geth conflict as a counterexample to this supposed inevitability remains deeply frustrating.
 Turns out the Crucible isn't a superweapon of Prothean design, it's an incomplete design that they inherited from the cycle before them, and indeed goes back almost (?) as long as the cycle itself. This makes no sense. Building a device based on an alien design without having a clear sense of what it does is a time-honored trope in SF--look at Contact--but you can't design a device whose ultimate form and function you don't understand, let alone start to design such a device, then expect a subsequent race could take a look at your unfinished designs and say, oh, here's what goes next. It literally makes no sense.
 Okay now we're doing stuff that's flatly in the realm of Space Magic. Mass Effect's worldbuilding has generally stayed within the realm of plausibility given their initial technical conceits, but now we're talking about evaluating every consciousness in the entire volume of the galaxy, deciding whether it's "synthetic" or not, and destroying it if so. The false dichotomy of organic vs. synthetic intelligence aside, this is just bullshit.
 Wait, no matter what I choose as my solution, the mass relays, the foundation of galactic civilization, will be destroyed? That can't be right.
 Wait, wasn't the Normandy on or around Earth, participating in the fight? What the fuck would Joker be doing between relays? Where was he going? Was he trying to escape the column of light? Why? The kid didn't say anything about damaging or destroying ships in proximity to the relays. If the Normandy was in danger, that meant the thousands of starships and millions of people in Earth orbit at the time were totally fucked. And based on everything we know about Joker, wouldn't he use the Normandy to fight to his absolute last gasp, rather than abandon Shepard and the rest of the crew? Why the fuck was he between relays?!
 Oh my god, Mass Effect 3, you are not pulling that trite-ass Adam and Eve bullshit. You are not. You are not gazing out over a virginal forest and suggesting that upon this fertile land you will live simply, this time, avoiding the mistakes of the past--not after I spent three games and hundreds of hours staunchly defending the basic idea that this civilization is worthwhile. You wouldn't just throw all that in my face. Would you? You wouldn't make an asshole out of me like that, would you?
 LIARA WAS IN MY PARTY DURING THE ASSAULT ON EARTH. How in the name of almighty gibbering fuck did she get on the Normandy?! LIARA T'SONI YOU GET BACK DOWN ON EARTH THIS INSTANT.
 So just to be absolutely clear, in a game whose core mechanics are meant to put meaningful narrative choice in the hands of the player, the ending boils down to choosing from one of three variations, none of which differ significantly from the others, and all of which involve totally flipping the table on the civilization you've sacrificed everything to protect. Remember how you undid Krogan sterilization, and now Wrex and Eve/Bakara can have kids? NOPE, WREX IS STUCK ON EARTH. Remember how you restored the Quarians to their homeworld, in exchange for which they brought their vast Migrant Fleet to aid you in your hour of need at Earth? WELL, NOW THEY'RE ALL STUCK THERE, AND NONE OF THEM WILL LIVE TO SEE THEIR HOMEWORLD REBUILT. Remember the Geth you saved? Remember EDI, the Normandy AI with the heart of gold? The only way to get an ending in which your Shepard doesn't canonically die is to choose to kill them all along with every other synthetic intelligence in the galaxy, regardless of whether it ever did anything to deserve death.
In a few minutes of cinematics, Mass Effect 3 manages to not only have a badly-executed ending that provides no sense of resolution or closure, but also renders every single decision you've ever made in the entire trilogy totally moot. It literally does not matter what you do.
99% of the way through this game, I was prepared to call it the greatest video game I'd ever played. Now I can barely look at it.