While technically HTML is not fundamentally a programming language, the tags it provides to manipulate content and functions within the constraints of a web server and browser to represent a webpage makes it the closest thing to one, and, on that note, it would be the easiest language to learn. It is literally open tag, then close tag. All you need to know is what tags do what and where, including any specific commands such as specifying an image's width and height in an image tag, for instance. This is as basic as it gets. HTML will be there regardless of what other programming languages you have on a webpage to give it functionality, so it may as well be called one since it "holds" everything in place.
Aside from that, an actual programming language, Visual Basic, is the next simplest thing. After this would have to be PHP, which uses a similar tag system as HTML with different syntax and many more functions to make a static webpage dynamic. After this, I would say Javascript or ASP.net are the next easiest languages, then Microsoft's C branches of coding, then whatever large enterprise datacenters use.