Agar.io

Brazilian developer Matheus Valadares built Agar.io in a few days in 2015 as a side project, and it ended up kicking off the entire ".io game" genre that browsers are still full of today. The rules are almost insultingly simple: you're a circular cell on a petri-dish map, and eating anything smaller makes you bigger.

The catch is that size is a trade-off, not a straight upgrade. A bigger cell moves slower and splits more easily under pressure, so top-ranked players deliberately eject mass to boost speed away from a threat, or split into multiple pieces to surround and swallow a smaller opponent from two directions at once. Staying near the edge of the map limits how many angles you can be attacked from, which matters more than raw size once several large cells start circling the same feeding zone. If the genre's stripped-down multiplayer format appeals to you, Paper.io 2 runs on the same "grow by risking everything" tension but trades cell-eating for territory painting.