I didn't see PHP mentioned here, so I just want to throw out there that you shouldn't underestimate how much legacy PHP code is out there to be maintained.
I think Python and Ruby both make you pretty employable, as well, and I prefer to work in those. But on the web, you're going to have to learn Javascript anyway, so might as well start with it.
And IMHO it's the best language and ecosystem you can get into today.
You will be able to be hired even in the highest paying companies.
Back-end? Python or Go, or maybe JavaScript