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Fear not Bat-dorks, Ben Affleck is going to be a GREAT Batman ... so says one of the most respected guys to ever don the cape and cowl, Michael Keaton.

Keaton -- who famously danced with the devil in the pale moonlight back in 1989 -- was leaving Toscana in Brentwood last night when he sang Ben's praises ... telling TMZ, "He's gonna be great."