In the Series, Tony killed and ordered many deaths. Paulie did a lot of hits. Silvio and Christopher did a lot. When Bompensiero gets it, everyone in there were the top hitters for the show. Corrado Soprano is another one up there. Then you have the tidbits of information like Paulie taking out a lot of heavy hitters in his time. Bobby Baccalieri Sr being known as the Terminator. Lou DiMaggio Galina doing a lot of hits. One person who does not get mentioned is Ercole DiMeo. You dont stay as boss that long without having to ok hits.
If we just go by what we watched in the series and not the tidbits of information we get during the show, then it would be Tony Soprano number one due to both him killing and ordering deaths, followed by Gualtieri. Either both Tony and Paulie are tied on screen killings or Paulie is ahead of Tony in that regard. Pretty much Paulie Gualtieri was Tony's number one hitman.