Mild spoilers in my review. If you've been paying attention to the pre-release press then you my review won't spoil anything for you.
This movie made the MCU feel like a true universe. More than any movie before it, and probably more than any movie after it, you felt like all these movies actually do take place together and are all connected in the most epic of ways. For someone like me who's seen all MCU movies and knows what happens in them, this is a huge huge gift. You're rewarded for your loyalty to the MCU. You feel like every moment is more impactful due to the movies you've seen. All those small bits they put into movies, like the de-ageing effects are used so damn beautifully. You even feel like small scenes from the first Ant Man are paid off.
It also brings into play my favourite story-telling element, which is time travel. I have loved time travel a lot, especially since I saw Back to the Future (a movie referenced in Endgame), and this movie makes such amazing use of it that you want to go and hug the makers of the movie. Characters have information their past versions don't, they are more aware and smart because of it. And you enjoy those moments so much because you've been with them as it happened.
I wasn't able to see the first Avengers movie in the cinema, but this movie gives me the gift of seeing that movie's most triumphant moment on the screen, and beautifully continues from the end of that movie. You feel like screaming with joy. That 2nd hour of the movie was the best part for me.
And the, of course, the final third, which gives you moments that fans have been waiting for since 2012. All those cast members coming together, that humongous end fight just overwhelms you. Heck, even smaller cast members (including one from the TV series) are in the movie. You keep hooting. It ultimately ends with the biggest pay off in the franchise. That very first Iron Man movie did something that changed super hero movies, and this movie brings a great moment from that one back.
Ultimately, you see the journeys of the original Avengers complete. Yeah, some may still be there and some may not, but you got to see them end their journeys with the most satisfying way.
Even more amazingly, one of my biggest complaints from the previous movies is solved here. We never got to see these Avengers just living their normal life, living in the same house, going on smaller mission. But we get that in the movie, and it's everything you wanted.
I don't think we'll ever see a cinematic event compete with this movie. Nothing can come close to what we've witnessed. Prepare for the opening weekend record to stay for a decade, maybe more, prepare to see every other big franchise pale compared to this. Just go and see this movie as soon as you can.
From now on, box office records will be Avengers: Endgame first, and then everything else.