The most recent studies that Brett contreras did showed that Decline Db presses activated more than any other pressing movement IIRC.
I never claimed that the flat bench press activates the most of all; I said it activates far more than machine moves. Also, the flat dumbbel press is better for triceps, anterior delt and trap thickness. Anyway, I do dumbbell presses and not flat bench for practical reasons - the bench always has a line to be used, so I use a regular bench and dumbbells instead of the flat bench.
SUCKMYMUSCLE