I think this comes down to your personal taste. You have no
contract with the producers so you can't use the usual "...used
with the permission of..." and since the public owns the clip
there is no need to "thank" anyone. If you want to credit the
writer, director, producer, DP or actor you could do that.
After all, you, as a member of the public, owns that film. You
can (as many have done) make a copy of the entire movie and
sell it. All without giving anyone credit.