BABYLON 5 with all the effects work redone for high definition. CSI MIAMI DEEP SPACE NINE THE NEXT GENERATION ENTERPRISE VOYAGER ANGEL BUFFY THE X-FILES
Since I'm limiting myself to film-based shows (keeping in mind that anything since the mid-1980s would have to have post-production redone):
I Love Lucy (with the option to recreate original broadcasts via seamless branching, commercials and all) Leave it to Beaver The Twilight Zone The Dick Van Dyke Show Bewitched I Dream of Jeannie Green Acres The Mary Tyler Moore Show The Waltons M*A*S*H The Bob Newhart Show Dallas Knots Landing Dynasty Hill Street Blues St. Elsewhere Cheers Newhart Designing Women The Simpsons
But how many of these would actually benefit all that much in HD though? I kinda doubt the quality of the film source for most of them would be all that great...
Twin Peaks Miami Vice Arrested Development (airs on HDNet currently, and looks great) Freaks and Geeks The Office (UK and US) Battlstar Galactica (both versions) Star Trek B5 Andy Richter Controls The Universe (not even on DVD yet)
Eh, I'll make a new post and update my list. A HUGE one though.
Quantum Leap THE FLASH Magnum, P.I. MacGyver StarGate SG1 BattleStar Galactica Doctor Who (1963-1989, 1995 film) Doctor Who (2005-2037) STAR TREK: TOS STAR TREK: TNG STAR TREK: DS9 STAR TREK: Voyager STAR TREK: Enterprise The Twilight Zone Mission: Impossible The A-Team TRANSFORMERS ThunderCats
Star Trek (The Original Series) Twilight Zone (ditto) Outer Limits (ditto) Sopranos
That's about it.
There are other series I love but they were shot on video (Monty Python, etc, etc) and others I enjoyed but not enough to own. Never got into the TV on DVD boom, I'm more into feature films.