It's wise to leave Jim and Edie out, cause I don't know how they wouldn't make every single list.
My list would have to start with the late, great Nancy Marchand. She played arguably the most colorful, indelible character in the whole series and always managed to live up to the fantastic writing that Chase and company gave her. If there were a measure for biggest impact per minute of screen time, I don't know how she could be surpassed.
In no particular order, here are the rest of my top 5:
Michael Imperioli. Can't really add much to what's already been said. His part called for a tremendous array of stuff, and he was rarely less than brilliant. I disliked his character for most of the series, but Michael never lost the ability to make me see some of the vulnerability, pitiful stupidity, and flat out humor of some of Chris' actions. I laugh my guts out in the scene when Chris comes to help Tony clean up the Ralph murder mess, and it's almost completely because of Michael's performance. He's equally as good in small scenes, like the one when he's rehearsing "The Glass Menagerie" for his acting class, and Adriana starts laughing. Just a remarkably complete performance that always came across with complete and utter authenticity.
Aida Turturo. Sometimes it's hard to separate how much of a character's success belongs to the actor and how much to the writing, especially when the character is inherently flamboyant, like Janice. That said, I just can't imagine any other actress in the world playing Janice. Aida was pitch perfect nearly every time she was onscreen -- hard to do with some of Janice's over-the-top behaviors -- and I easily rank her acting contributions as among the show's best.
Vince Curatola. I know he wasn't an actor before the Sopranos, but he proved that, with good casting, talent goes a long, long way in front of the camera. It takes some kind of innate charisma to share the screen with Gandolfini's Tony and make you actually feel the "superiority" that Johnny and his NY gang represented versus their Jersey counterparts. Johnny's sitdown with Tony in "All Due Respect" ranks IMO as one of the best, if not the best, mob/business scenes in the whole series, and it was all predicated on Vince's ability to convey how easily Johnny would be able to crush Tony if his demands weren't met.
Joe Pantoliano. There were many fine performances given by actors playing Tony's adversaries, including the great characterization of Richie by David Proval. In the end, though, Ralph was a bit more complex and layered, and I thought his culminating performance in "Whoever Did This" was flawless as it was compelling.
There was rarely a performance on the show that wasn't good, and I feel a little guilty leaving out the names of actors who provided so many good moments during the series. Chase always credited the talented cast as being his favorite aspect of working on the show and the factor that really brought it all together. I'm a writer's fan first, but I certainly respect that the same scripts with a less uniformly superb bunch of actors would have resulted in a different and lesser show.
Tony, his spirits crushed after b-lining to the fridge first thing in the morning: "Who ate the last piece of cake?"