For me, hands down, the Raffica. I have never had any secondary that i liked more than the raffica from MW2, that goes for any FPS. I believe that it would have been a great counter to the akimbos in MW3... thoughts? what is your favorite machine pistol or secondary in the MW series?
My personal favorite was the TMP BUTTTTT, the best were the G18 akimbo in mw2 and the FMG akimbo in mw3
The Raffica was also my favourite, but I suppose the most effective would be the G18 and FMG9 as Dedge points out.
it's hard to beat the Raffica. It played it's part perfectly for me, It was my pistol at close range and a more reliable smg for long range due to it's recoil.