Putting aside my own personal preferences and trying to look at it objectively:
First off, how can you name one single show, given all the different genres over the years? Can you really compare the best sitcom to the best drama to the best news program?
Second, how do you define best? By your personal preference? By longevity? By ratings? By critical acclaim?
I think you need to look at programs the way you look at almost any art: a balancing of critical acclaim, commercial appeal, influence on others and longevity.
Using that standard, I'd say that (while it's not a personal favorite of mine) the best sitcom is "I Love Lucy." It pretty much invented the genre (both creatively and technically). It was a critical and commerical success and it is still enjoyed by fans to this day (on Nick at Nite).
Similarly, I'd say the best news program was "60 Minutes," hands down. Every thing else is an imitation and it's still the number one news mag around.
As far as dramas go, that's a little trickier. I'm not sure there was one groundbreaking drama, that one huge hit that wrote the rules the way that Lucy and 60 Minutes did and is still popular today.
I'm tempted to say "Hill St. Blues." It was the first drama to bring a true cinematic sensibility to TV drama, with hand held cameras, overlapping, Mametesque dialogue and better than average production values). It was wildly influential, with its use of multiple story lines and a varied cast of gritty characters (shows as divergent as "the West Wing," "the Sopranos" "24" and "Alias" are owe a debt to HSB and the way that storylines were allowed to continue from week to week, major characters were allowed to change or die and heroes could be cast in shades of grey). And it was a commerical and critical success.
Other thoughts?
