here are some of the most popular IO games of 2021 based on player engagement and user reviews:

  1. Agar.io – This game is one of the pioneers of IO games, and it’s still a fan favorite. The gameplay is simple yet addictive, and the objective is to eat other players to grow larger while avoiding being eaten.
  2. Among Us – While Among Us was originally released in 2018, it became a massive hit in 2020 and continued to dominate in 2021. In this game, players work together to identify an imposter among the crew.
  3. Slither.io – Similar to Agar.io, the objective of this game is to eat other players to grow larger while avoiding being eaten. However, in Slither.io, players control a snake-like character instead of a blob.
  4. Krunker.io – This is a first-person shooter game with a retro feel. Players can choose from different characters and weapons to compete against other players in various game modes.
  5. Zombs.io – This is a survival game where players build a base and defend it against hordes of zombies that attack at night. The game has a resource-gathering element that makes it more challenging.
  6. Wormate.io – In this game, players control a worm-like character and try to eat as much food as possible to grow larger. The game has different skins and power-ups to make gameplay more exciting.
  7. Paper.io – Players control a character that leaves a trail of ink as it moves around the game board. The objective is to capture as much of the board as possible by enclosing areas with your trail while avoiding being killed by other players.

These are just a few examples of the best IO games of 2021. There are many other great games out there, and new ones are constantly being developed, so it’s worth exploring different options to find the one that’s right for you.