James Gandolfini, The Sopranos. I remember watching that show pretty much every Sunday. I can't think of a better actor, he pretty much owned that role and hit every note and range of emotion. Plus, I might be a bit biased because he is also from Jersey, and he also helped out the Wounded Warrior's Project, which I also support and think are just great. But I really think that he might have given the all-time GREATEST performance in any television series ever! I also hear that Martin Sheen was fantastic in The West Wing as well.
Fore sure Gandolfini is the best dramatic role in TV history. But I always consider it a show of the 2000s.
For the 80s, the breakout TV role was Alex P. Keaton by Michael J. Fox. "Family Ties" was designed to be a star vehicle for the parents, but Fox stole the show.
For the 90s, it's somewhat harder to define such a singular TV performance that defined the decade. But I'd go with either Michael Richards as Kramer or Jason Alexander as George from "Seinfeld."