The best one is NPM. You have to pick two identical icons that are overlayed on other images. But if the username you picked is taken then the whole form resets

The best one is NPM. You have to pick two identical icons that are overlayed on other images. But if the username you picked is taken then the whole form resets, and you have to solve the captcha again. There is no way to check if a username is taken before solving the CAPTCHA, even though npm usernames are public.