What's the most intrusive way a website can track who I am?

I figured programmers here make systems like that and dissect them, so this is probably the right section.

What is the most intrusive way a website can identify me uniquely, and keep that information even if I delete my cookies and cache?

Bonus: is there any way to prevent it besides VPNs, proxies and such?

P.S. I know that IP is public and is highly identifiable unless you're using public wifi, so let's just exclude that for the argument's sake.