Wormax.io is one of the best if not the best Slither.io alternative. If you like slither you will love wormax.io
The gameplay is pretty much similar to slither.io where you eat pellets to grow bigger and longer. You can also eat other players pelets when they get destroyed. Players get killed when their snake's head touches the body of another snake or worm.
But there are a lot of new features present here. Features like new skills, and lots of skins - you can even play as PewDiePie here - make this game a very enjoyable experience. Skills include using nitro to speed up or slowing down or stopping completely. It's like a souped-up version of Slither.
You can control the snake using your mouse. You can also download it on your phone or tablet from the Googleplay or itunes store. Enjoy.