Spencer Pratt is having the best Memorial Day ... thanks to Ben Affleck.
The former 'Hills' star apparently had a chance encounter with Batman Monday, and he's totally stoked about it. He shared a vid with the actor and wasn't shy about gushing over him ... saying he had chills from the meeting.
Spencer was pretty quick to point out he's been starstruck before, though -- recently -- by another huge celeb ... but Ben didn't seem to mind.
Affleck's got reason to be in good spirits, too -- he's moving into his new $19 mil Pacific Palisades mansion ... and things look to be going great with his gf, Lindsay Shookus.