Which ammo has the greatest muzzle flash? Something you can see in broad daylight.

The Federal XM193 loads, being pull power, often have a pretty bright flash. This could be reduced if flash retardant was added. A few of the commercial loads also have pretty bright flash. Personally, I rarely notice much in the way of flash at all from any of the loads during the day. But in low light or complete darkness, that's when it becomes really apparent.

-Charging Handle

If you really want to have some fun try some 7.62X54R 177fmj from a M-44 Mosin-Nagant.  This load got the bee's in the rafters thinking about moving to a new home!  It was a huge fireball even in bright daylight.  It is also very,very loud.  I don't see how anyone would have any hearing left if you were in a trench with 100 guys on either side of you shooting these things, not that that is really important at that time.