I don't watch a lot of TV these days (I can't stand dubbed TV shows), but I'd say that our favourite (almost) current show was 'Lost.' We recently bought and watched the whole series on Blu-Ray (man, was Hawaii ever gorgeous in HD!!!).
In the 90's it was 'The X-Files,' (we have that all on DVD) Babylon 5, ST:NG, ST: DS9).
In the 80's it was hmmmm.... a bit of a blur, due to being a professional, impoverished student with no cable...... Moonlighting, Nightcourt, probably a couple of others.
In the 70's it was 'M*A*S*H, All In The Family, probably a couple more.
In the 60's, it was Batman (gotta love that Bat Shark Repellant!).
Special mention to 'Father Ted,' which my wife introduced me to sometime back. I can watch that over and over and still almost have an accident from lol'ing so hard.