I would say javascript is by far the best, not because of the language itself, but because it lets them work and modify something they're already using a lot : the web and a browser.

A close second could be swift for the same reason ( and also because you can start smoothly with just drag n dropping on a storyboard, then slowly add dynamic behavior with code).

Processing is fantastic, and it was easy to segue from processing to Eclipse in just a few lessons (it's about 20 lines of code)