AND1's 'The Professor' Says James 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/stare down 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 artist ... and who he feels is the best in the game today (and trust us -- it's a shocker).