Masterpiece!

Rating: 9/10 (14 October 2019)

Avengers End Game is about sacrifices. The first Avengers movie was about the challenges of having power. Age of Ultron was about unintended consequences. Infinity War brought those two themes together when almost the entire Marvel Cinematic Universe was faced with the challenge of the mad god Thanos. Endgame starts in the aftermath of that event.
The initial mood is one of sorrow and remorse. Stark for example, talks about "losing the kid" aka Tony Parker-Spider Man (Tom Holland). There are support groups for people to talk about their losses, which features a cameo by Joe Russo, one of the co-directors. Clint Burton (Jeremy Renner) was even turned into a killer as a result. Everyone was completely devastated by Thanos.
Sacrifice is then introduced when Scott Lang (Paul Rudd) returns from the Qauntum Realm and tells the Avengers that time operates differently there. If they could manipulate the realm, they might be able to go back in time and reverse Thanos' act. Different characters then have to give up something to carry out this task. Tony Stark for instance, has to leave not only his wife Pepper Potts (Gwyneth Paltrow), but his young daughter as well. One of the team has to sacrifice themselves to attain one of the Infinity Stones. Others pay in different ways.
Going to the past also allowed Marvel to pay homage to its previous movies since this marked the end of that whole first generation. The first place visited is New York City when the Avengers were formed and fought off Loki (Tom Hiddleston) and the Chitauri. There, Banner runs into the Ancient One (Tilda Swinton), and Stark and Captain America (Chris Evans) come across a bunch of Hydra agents working inside SHIELD that were seen during Captain America Winter Soldier including Alexander Pierce (Robert Redford). Then Thor (Chris Hemsworth) and Rocket (Bradley Cooper) go to Asgard during Thor: The Dark World where the Thunder God gets to reconnect with his mother Frigga (Rene Russo) James Rhodes (Don Cheadle) and Nebula (Karen Gillan) visit Morag from the first Guardians of the Galaxy, while Natasha Romanoff (Scarlett Johansson) and Barton go to Vormir seen in Infinity War. There's even one extra flashback where Stark gets to meet his father (John Slattery) and Cap sees Peggy Carter (Hayley Atwell), which is some foreshadowing.
The film is still able to throw some laughs in despite the general feeling of loss. Scott Lang for example, says they should be able to travel through time as long as they don't break the rules, which come from Back to the Future. Then when they experiment Scott gets transformed into a pre-teen, an old man, and then a baby. Bruce Banner (Mark Ruffalo) declares that a win. What really steals the show however is the depressed Thor. That is one of the funniest things I've seen. There's a great line where Stark says Thor reminds him of Jeff Bridges from the Big Lebowski. It turns out Thor has some problems with players on Fornite as well that's worth some laughs.
Of course, there's a big giant battle at the end that has some fantastic parts such as a whole army coming to the rescue to back up the Avengers against Thano's forces. All the female heroes forming up to push through the bad guys, and a great exchange between Cap and Thor.
Really, every Marvel movie up to this point was leading up to Endgame and it delivered.