What's your endgame?

If you want to use this as a tool to get a new job, I'd recommend leetcode above all others.

If you just want a fun way to learn a new language's syntax, I enjoy grinding the easier problems in CodeWars.

If you're looking to get into competitive programming, I've heard good things about Codeforces, CodeChef, and TopCoder.

I personally like https://www.codeeval.com/. It's not competitive really, just solving coding problems. But I find it great especially for learning new languages or refreshing myself on old ones.