From the past-

Bonanza
Ed Sullivan
Dr Kildare
Smother's Bros
TWTWTW
Twilight Zone

In the 70s, I was out of the country, and didn't watch much telly. On my return I caught up with;

Star Trek
Monty Python
Masterpiece Theatre

And more recently-

Star Trek NG
Babylon 5
The Simpsons
Buffy/Angel
The Office
West Wing
6 Feet Under
ER

Funnily enough, I watch far more TV here in England, than I did back in the States, and most of them are American Shows at least six months old! The state of British Television has sadly declined to shows on cooking, DYI, gardening, and far too many "real people doing crap things" for me to count.

PS-Not asked for but I'm including it here anyway...the ALL-TIME WORST TV program I can ever remember seeing was "Boudicca".