Harden's Crossover 'Not the Greatest'

... Here's Why

James Harden's crossover broke the internet -- and Wesley Johnson's ankles -- Wednesday night ... but one of the best ball-handlers on the planet says he's seen way better!!

We got AND1 legend "The Professor" -- real name Grayson Boucher -- the morning after Harden's dirty cross/staredown combo vs. the Clippers ... and he told TMZ Sports why he thinks it should NOT go down with the all-time greats.

Don't get it twisted -- Professor says the Beard's shake was "one of the best of the season, for sure" ... but to get an A+ in his class, you gotta drop fools with Allen Iverson-level sauce.

Speaking of A.I., Grayson told us whether or not Iverson's his pick for GOAT NBA crossover…