Which cast member had the most successful career after this ended?

I think the obvious pick here is James Gandolfini. Before the Sopranos he was barely known. James appeared in many great films following his Tony Soprano days.

Edie Falco was in Avatar 2

Michael Imperioli recently appeared on White Lotus

Without question it's James Gandolfini. He went from being an unknown bit player to a famous actor. No one else in the show had any kind of fame or success afterwards.

Joey Pantoliano is still a working actor and well into his 70s today.

When you really look at the resumes, nobody really had an incredibly successful career afterwards. James Gandolfini was in several movies, but nothing really great (of course his career was cut short); Edie Falco was probably the most successful as she won another Emmy for Nurse Jackie; Michael Imperioli has had a nice career that 99.99% of actors would kill for, including Broadway. Matt Servitto has had a nice career as well and so has Steve Buscemi.

But now that I think of it the only answer is Lady GaGa.