What are your all time memorable moments in the ME universe?

For me …

When Shepard climbs out of the rubble on the Citadel, after the piece of Sovereign hits the tower.

After Shepard's air bleeds out, the piano chords strike and the ME2 logo pops up.

After the Normandy departs Earth, to go get help for the Reaper attack, the piano chords strike and the ME3 logo pops up.

When Drack talks about how having to raise Kesh saved his life … and when Ryder chews Liam's butt for being such an idiot.

the conversation between Shepard and Harbinger in Arrival
watching the reapers fall over destroyed
seeing the SR2 the first time
going through the Omega 4 relay

This choice never really did it for me, even in my first PT. Maybe because, in that first game, I assumed whichever person I sent with the salarians would be the one to die. Then, that didn't happen. I actually had another opportunity to choose, but the second one just feels so ridiculous because you've got geth incoming at both locations and Shepard just stands there for several minutes doing nothing. Then, once you finally make up your mind what you're going to do, it takes forever to get back to the bomb or move forward on to the tower. The set up just fell flat.

Most memorable for me was Grunt surviving in ME3 because, after just losing Mordin and Thane on back to back missions, if Grunt had also died I would have rage quit then and there and probably never finished the game the first time, let alone the 20 some odd times I ended up playing it.

They should've kept the difficult third route that let you save both rather than get rid of it to make drama for drama's sake. 

Ah yes, saving A/K. It would have been easy to happen. If all squadmates have been recruited, send the two squadmates that aren't being used to rescue A/K while Shepard rescues A/K. If not, have the 2 soldiers standing guard on the SR1 go get A/K.

What role would they play if both survive? At the beginning of ME2, Miranda ask that question to Shepard. He/she says that after dealing with Saren, A/K was reassigned to another unit. A/K will be the one that is protecting the council in ME3 while A/K, who wasn't reassigned, stays on the SR2 for all of ME3.

- The opening of ME1. I had read a lot about the game, seen plenty of trailers and engaged in conversation on the BW forums but I managed not to get too spoiled. (I got the game on PC, so it was already out for a while on XBox at the time.)
To top things off, a few weeks before the PC release, "Evil" Chris Priestly had a give-away for a few copies on the forums and I actually won one (I think this is the only time in my life I ever won in any of those things ). I had also bought and read Revelation before the game was out on PC. Long story short, I was completely hyped up about the game.

And boy did it deliver. First the character creation process (the Alliance personal computer thing) that was already so immersive it reminded me of the old Westwood installers like the EVA installer for the first Command and Conquer game). Then the intro scene with the Normandy SR1 (IMO still the most beautiful space ship ever designed) going through the relay. The music, the voice over, it all was marvelous!

- First arrival at the Citadel, with everyone still being in awe of everything, me included.

- The Virmire mission. I agree with operationathena , that mission had it all. The pacing was great, the interlude in the salarian camp, including the Wrex dialogue and "Hold the Line!" was very cool. I thought the music during the base assault was one of the best tracks in the game. Fighting through the base is also lot's of fun and has IMO some of the best level design in the game. Then we learn about indoctrination and finally meet Sovereign. All the pieces come together. It was brilliant. I do agree with some others here that the A/K decision had it's downsides but still, it was a dramatic moment and during the first playthrough I had no issues with the narrative. Than the showdown with Saren himself and hearing his side of the story (and that there was some twisted logic in it). Virmire just combined all the best about ME1 and put it out there in condensed form.

- The battle of the Citadel, Joker having a couple of real badass moments and walking up the Citadel tower. The ME1 finale, at the time, was storytelling as I had never experienced it in a video game before.

- Watching the ME2 launch trailer for the first time. Best trailer ever made, IMO.

- I bitch a lot about ME2 here on the boards, especially its beginning, but even I could not resist the feels when the SR2 was revealed. As an emotional roller coaster, the ME2 intro (which I consider to last until we get access to the galaxy map) was certainly executed very very well.

- I would love to put the "Garrus is Archangel" reveal here but unfortunately, I got spoiled on this one. One of the worst spoilers ever to happen to me. I'm sure it would have been a great scene if I had been ignorant going in.

- The suicide mission. On my first runthrough of ME2, I lost Legion (since I was loyal to Tali but did use him for the vents). That gave it just the right amount of drama. That mission is just great, all those choices and consequences. Again, the music does a lot in this one.

- The end of the Overlord DLC. My eyes got misty for sure. Never enjoyed paragon-slapping someone more.

- First time playing a renegade Shep through ME2 and leaving Zaeed to burn on Zorya. What a scene.

- ME3 did not really grip me until Menae. Maybe it was because we saw too much of the earth mission before release (and Mars, I found very confusing at first). But Menae to me really drove home the atmosphere this game would have, with destroyed Palaven overhead, the space battle going on in the dark skies above us and our squad quite literally in the trenches with the turians. Top notch! Also, getting Garrus back yet again was great!

- Tuchanka, basically the entire final mission of that ark. There is just so much going on, so many things are taken into account and here the game really manages to capitalize on the entire trilogy setup big time. And again, the music plays a significant role here.

- Liara's time capsule scene. I know many people dislike Liara, especially in ME3 but not me (I do like her and she is the romance option for my main Shepard throughout the trilogy, even ME2). To me, the time capsule scene is simply beautiful and it drives home the calamity that we are in. It's both sweet and melancholic.

- Shooting bottles with Garrus. This little scene works perfectly, no matter if it's a bro scene or a romance scene, no matter if male Shep or femShep. Whoever wrote this is a genius. all the lines (as well as the deliveries and the cinematic pacing) are perfect. Hollywood could not have done this better.

- Rage after Thessia. It was cheap, it was infuriating (taking player control away like this), the character of Kai Leng is just stupid but I will say this, they did succeed in one thing: After Thessia, I - as a player - hated that guy and really REALLY wanted to kill him. I didn't roleplay Shepard to want to kill this bastard, no, I myself wanted to see him dead. Doesn't happen often with fiction to me to have such a strong reaction, so kudos on that at least. Corresponding to this, on Chronos Station, the "this is for Thane you son of a bitch" worked, again, the scene is somewhat stupid but it felt fucking good.

- The fleets arrive at earth. This is the only time, I can remember when I - again, me the player - sat in front of my computer, bumping my fist into my palm and saying out loud "Let's get'em guys!" The arrival of Hammer and Sword, the feeling that the last five years of Mass Effect had all lead up to this point, and - knowing what happened in the suicide mission last game as well as Tuchanka and Rannoch - that it will all come to fruition now one way or another, that was something else. (Of course, the ending did not quite offer what I expected there in terms of reactivity to past decisions but I didn't know that yet during my first unspoiled playthrough.)

- Some of the goodbye scenes at the FOB. This started out very weird (the conveyor belt goodbyes with the comm calls) but once you get to the room with Garrus and the Primarch, it delivered. Garrus and Liara have especially heartwarming scenes but Wrex, Tali, Edi and even Javik also have some great lines there.

- The end of the Citadel DLC when played as epilogue (thanks to the Citadel Epilogue Mod). Finally, a perfect ending to a masterpiece of storytelling.
- The real time shuttle ride from Hyperion to Habitat 7 in the beginning of ME Andromeda. (I was impressed that they made that work and feel seamless in the engine)

- The deployment of Prodromos, when you realize that building these outposts is gonna be a real thing in the game and that we'll be able to walk through them and talk to people there.

- The finale of Mass Effect Andromeda.