Great to see some love for Homicide:Life on the Street in this thread. It's one of the more overlooked yet seminal classics of all time, imho.
To it, I'd add:
The Wire
Deadwood
Breaking Bad
Mad Men
Six Feet Under
Seinfeld
The Sopranos
Hill Street Blues
Early Simpsons
In pretty much that order, too. If counting mini-series then add Band of Brothers and The Singing Detective to that list.
There are plenty of other series that were/are really good and come close, but this is the HOF we're tawking 'bout here.