That's why I said "arguably", of course.

But, to be honest: Forth is great, but... well... there's no "easy mode" Forth. Whereas there is "easy mode" Scheme - even SICP starts with a very simple and gentle up ramp - so I'd argue to give the honours to Lisp (I'm putting Smalltalk in second place). But that's my personal opinion, I get to put it on my book cover, that's all :-)