I haven't seen it but since James Gandolfini passed this week I would say The Sopranos- I heard a lot of pundits say this week that it changed the way dramas have been on television since and I would agree. With Tony Soprano, we wouldn't have had an Olivia Pope or Gregory House. We'd still be stuck with shows that are either too sappy or too dry with routine.
Also I watched ER growing up and it was one of my favorite shows. The only thing I will say that bothered me was that the quality decreased and it stayed on too long. I feel like once they were swapping characters in and out, it got stale but maybe you will think differently.