Club Penguin. Hands down, without a shadow of a doubt.
On a more serious note, there are games like Counter strike:Global Offensive which is okay for casuals but mainly for competitive players. Most of the people in the CS:GO community range from pretty nice to incredibly toxic. Runescape is also a decent choice. I've played it before and I really liked it when I was playing. It's hard to get into it but it gets incredibly fun after you have developed a sense of the environment of Runescape. But the negative part is the graphics (It came out in 2002~ if I'm not wrong), grinding and pretty much the Pay-to-play features. I've played Combat Arms as well. I really loved it when I played it a few years ago, but now.. It's pretty crappy.
Note; I'm only recommending you games that I've played before and enjoyed it.
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theres overwatch which is a fun multiplayer game if you played with friends. There are also league which is a moba game. Theres also csgo which is a fps game. And World of Warship which is a fun multiplayer battleship game.