Click on the green lock symbol in Chrome to see what certificates each site uses.

Google issues its own, GitHub uses DigiCert(http://www.digicert.com), Hacker News uses Entrust (http://www.entrust.net).

In general, Verisign (http://www.verisign.com/) will be the most expensive and presumably the most widely supported, but there's no need to pay up for it when DigiCert will work just as well.