I dont know how these names couldnt be at the top if you are talking about actually doing the hit. If history is correct then these guys are at the top of the list we are speaking of: (everyone has already said these, just adding a few of my own)

Roy Demao
Tommy Karate
Jimmy Burke
Lefty Ruggerio "I got 26 hits under my belt"
Scarpa SR
Charlie Carnegila
tony Spilatro
Bobby Borrelio
Anthony Capo (admitted to around 15 murders as the trigger man I think for the decavalcantes, not sure though, could be wrong)
Tommy Desimone
Frank Sheehan
Salvie testa

Those are the names I can think of as "trigger men" but im sure im off base on a few. Like Gravano, he says he only pulled the trigger once but pled to 19 murders as the set up guy, so who freakin knows