What is the best version of HTML to learn?

I'm looking to learn to program in HTML but unsure on what version to use.
HTML4
HTML5
XHTML
Which one of these are the best to learn or is there any others (these are the only ones i have heard of)?

Any Help! Thanks!

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).