2015's Ant-Man still has the dubious honor of having the worst post-credits scene in the Marvel Cinematic Universe to date. Although post-credits scenes aren't exactly a new idea in cinema, Marvel has turned them into a core part of the brand. Every film in the MCU has a post-credits scene, and some have quite a few mid-credits scenes to boot. Last year's Guardians of the Galaxy Vol. 2 broke all Marvel records, with no less than five mid- and post-credits sequences.
Sometimes these scenes are just added for fun; that was the case with several in Guardians of the Galaxy Vol. 2. Other times, they're intended to foreshadow future movies - a prime example being the post-credits scene of Avengers: Infinity War, which prepared the way for Captain Marvel to enter the MCU next year.
But looking through all the mid- and post-credits scenes to date, there's one that stands out as the worst. Sadly, it's the post-credits scene for Ant-Man. The sequence is actually a scene from Captain America: Civil War, showing Steve Rogers and Sam Wilson standing over the defeated Winter Soldier. They consider calling Iron Man for backup, but decide he may not be able to help due to "the Accords." When Steve concludes that they're on their own, Sam disagrees. "I know a guy," he says, and viewers are expected to (correctly) assume he's referring to Ant-Man. This post-credits scene is clearly intended to reassure viewers that Ant-Man will return in 2016's Captain America: Civil War.
Unfortunately, it doesn't make any sense at all. The post-credits scene drops you into the middle of another movie, referencing events that haven't actually been explained. The "Accords" are yet to be established, and the focus on Steve Rogers and Sam Wilson on the run supersede the Ant-Man setup. Making matters worse, the post-credits scene is actually an edited-down version of the full scene - and the dialogue doesn't even work in the context of Captain America: Civil War. Sam and Steve aren't "on their own" at all; Sharon Carter is able to help by giving them their weapons back, and Cap then calls in Hawkeye and Scarlet Witch as well as Ant-Man.
A much better alternative would have been the scene where Scott was introduced to Captain America for the first time, a forty-second-or-so bit of in-tone comedy that doesn't give up too much of Captain America: Civil War's plot. Or, if showing who was in Team Cap turned out to be too spoilery, a scene of him being "recruited" by Hawkeye or Wanda would have been easy enough to shoot and again promised his return.
The next year would see Marvel repeat this approach with Doctor Strange, where a mid-credits scene showed the future Sorcerer Supreme in conversation with Thor. But this one made a lot more sense; the dialogue clearly established the narrative of what was happening, and the nature of Doctor Strange's cameo in Thor: Ragnarok was signposted effectively. While it was still a clip from another film, it worked. Ant-Man's, in contrast, simply didn't.
The irony is that Ant-Man also featured a mid-credits scene that would have served perfectly well as a substitute. It showed Hank Pym decide that the time was ripe for Hope to suit up as the Wasp at last. Given Hope is co-star of the next film in this franchise, that's a powerful and effective teaser in its own right. Using this as the post-credits scene would have emphasized Wasp's importance, making the setup even more noteworthy.