Common -- Snoop Dogg is NOT the Best Rapper/Actor ... But I Know Who is

Common says Snoop Dogg was barking up the wrong tree when he declared himself the world's greatest rapper/actor ... 'cause he tells TMZ, someone else deserves the #1 spot.

The "Hell on Wheels" star was heading home from the gym on Monday ... and we asked him about Snoop crowning himself king of the actor MC's.

Common says Snoop's leaving out at least one famous rapper who kills it on the big screen ... and he took the high road by not naming himself.