Paper.io

A territory game with one rule that does all the work: you claim the area enclosed by the loop you draw, and while you are outside your own territory that trailing line can be cut by anybody, including you. So every claim is a calculated exposure, and the size of the loop is exactly proportional to the risk you took making it. That tension makes the optimal play clear and hard to follow. Small loops are safe and slow; large ones are where the score is, and the longer you are out drawing one the more likely somebody crosses your line. Experienced players claim in shallow sweeps along the edges of their existing ground rather than making dramatic excursions, and they cut rivals' lines as a primary tactic instead of expanding at all. Rounds are short and restarting is immediate, with real players rather than bots filling most lobbies. The presentation is flat, bright and perfectly legible, which is all it needs to be. It plays one-handed on a phone, runs on anything, and explains itself in about ten seconds — which is why this format spread as far as it did.