Hands down the best video game ever made. To date. It is not like most other games on the market, and therefore lacks appeal to many gamers.
The problem is that Sea of Thieves is a game where it is ultimately up to the player to create their own fun. I don't think it's possible to really enjoy playing long term without getting creative with what you do when you play.
The best analogy I can think of is that most games resemble an amusement park. All the fun is meticulously and professionally engineered. Even in non-linear games, a lot of what you do while playing is designed and provided. Sea of Thieves is more like visiting a State Park. There's a forest strewn with trails and a place to BBQ and/or a beach. People that visit bring the fun with them.
Sadly most gamers either aren't capable of grasping this idea or aren't interested in spending enough time in a seemingly empty(ish) game to figure it out. Until more games follow this pattern, our culture probably will never fully embrace SoT. For now, for those of us that "get it," it is far, far, far and away the best game on the market.