The Wire is number one for me, and it's not remotely close.
After that 6 Feet Under, Breaking Bad, Sopranos, Mad Men, Seinfeld, Band of Brothers, etc. Splitting hairs to rank all the serial dramas against each other.
Top Serials Shows:
Homicide Life on the Streets (up to season five)
The Wire (season five didn't happen)
Wire in the Blood