Twenty-one is correct. You use the words, one, two, three, when you write something formal, like essays or newspapers. In informal settings, like online forums, you can use the numbers 1, 2, 3.

I'd play it safe and keep the hyphen. For example, "twenty one liners" can mean two very different things: 21 cruise boats, or 20 witty sentences. It's a much better habit to write numbers as words, unless the numbers are definitely technical, or very large. Staying with (universal) numerals just creates a hole in your learning - that is, when you see a number you think of the word in your native language and not your target language.

Twenty-one, but both will get the meaning across. Even if you use the wrong one, nobody will think that the grammar is bad anyways.