Here are some of the best IO games, based on their popularity and player reviews:

Agar.io: One of the most well-known IO games, where players control a cell and aim to grow by consuming smaller cells while avoiding larger ones.
Slither.io: Players control a snake, eating pellets to grow longer while avoiding collisions with other snakes.
Diep.io: A tank game where players destroy objects and other players to level up and upgrade their tanks.
Krunker.io: A fast-paced first-person shooter with various game modes and customization options.
Paper.io: The objective is to capture territory by drawing lines and enclosing areas while preventing others from doing the same.
Surviv.io: A battle royale game where players scavenge for weapons and supplies while trying to be the last one standing.
Zombs.io: A tower defense game with elements of resource gathering and base building to defend against waves of zombies.
ShellShockers.io: An egg-themed first-person shooter where players control armed eggs in a free-for-all or team-based combat.
WormsZone.io: Similar to Slither.io but with more customization options and power-ups.
Mope.io: A game where players start as a small animal and eat food to evolve into stronger creatures while avoiding predators.
These games are known for their addictive gameplay, simple mechanics, and competitive multiplayer modes.