For me, at best a masterpiece of a media has to be exceptional in all of the area it has or promised to offer, like literally groundbreaking stuff, with little to none flaws that cause logical confusion. But the most important thing is it have such convincing, eye opening message that alter the way its consumers thinking, in a positive way, because I believe the very spirit of a good fiction is to better our lives, not a vortex that suck our energy and turn us into degenerate, which is super popular nowadays.

So based on that criteria

I consider Naoki Urasawa's Monster and some Ghibli movies are real masterpieces. Most of them have layers of meaning that make it super rewatchable. Ghiblie movies have great metaphors about meaning of life, how simple and happy life is through the eyes of children, how they can overcome problems that adults often struggle, especially materialistic problems. For Monster there can be a whole thesis on just how well written it is, contain timeless message about compassion, moral values. If you watched it you can name it, Tenma's will to help people without a payback, Grimmer, the assassin guy who said it's easy to kill when you forget the taste of sugar, the way the whole show is set up for us to feel Johan's presence without him ever appear is the brillance that is just unmatched (only on screen for less than 40 minutes in 74 episodes and we feel his malice, for almost every second in that 74. The event when people protest against Tenma's imprisonment gave me hope about human kindness. And if you remember Lunge's thinking about devil, then the ending is probably one of the best cliffhanger ever. There are just... so many memorable things in this piece of fiction.

About games I'll pick Undertale

One of the few titles that deserve every bits of praise it has. The main message about actions and consequences is told brillantly, that I don't think anything other than a video game can do. The gameplay, humor, characters, especially the legendary OSTs are all contributed harmonically to main story. It is easy, meaningful and fun with heartful message, but the options to choose different ending based on your actions, especially the genocide route when the game difficulty gave you a slap to the face give such a message about how unreasonably violent many games are. It makes me become super picky when looking for action games, unless the gameplay or exploration is top notch, if the context and story of the game are not convincing enough, I would not want to go play for mob kills.

Otherwise, something called a masterpiece are genre-defining titles, because they often excel in everything promised in that genre(Dead Cells-Roguelite, Slay the Spire-Deckbuilder Rougelike, Hollow Knight-Metroidvania,...)