Otherwise, it flops.
Law & Order: Jill Hennessy (lawyer Claire); Law & Order has gone seriously downhill without its resident Canadian, but Crossing Jordan got the benefit.
And Rene Balcer, who runs the L&O show these days, of course. Various Cdn actors have been on episodes: Sarah Botsford, Megan Followes (another one to add, better known in the US as Ann of Green Gables than as the murderer whom Jack McCoy tricked the bow-tied dullard Cdn ambassador into returning to the US to be executed).
Homicide: Clark Johnson (Meldrick Lewis); he was previously in Cdn prime-time drama E.N.G.; his sister Taborah acts, and Molly sings.
Ally McBeal: Gil Bellows (Billy Thomas, her leading-man ex who died of a brain tumour)
Alias: Victor Garber
24: Kiefer Sutherland, of course, but also Elisha Cuthbert (who plays his daughter), Leslie Hope (Teri Bauer), Alberta Watson (Erin Driscoll) and Mia Kirshner (the one who blew up the jet); Alberta Driscoll was also Madeline in Nikita.
Interesting Kiefer tidbits:
http://www.topsynergy.com/famous/Kiefer_Sutherland.aspDuring the fall of 2001, Sutherland unintentionally interrupted the filming of the premiere episode of the online series The Lonely Island. In the episode "White Power", The Dudes beat an old lady and steal her purse to facilitate their addiction to teeth whiteners. Sutherland, driving by at the time, jumped out of his car and intervened, only to discover that he was interrupting a student film. A portion of Sutherland's appearance is displayed after the credits.
Scrubs: Sarah Chalke (Elliott); also in Roseanne (the replacement daughter)
St. Elsewhere: Bruce Greenwood (the bad guy), also the star of Nowhere Man and a nework exec character in Larry Sanders. St. Elsewhere also had Howie Mandel, of course.
Larry Sanders: Scott Thompson
Family Ties and that other one: you listed Michael J. Fox, but I'll throw him in too, as evidence of the theory; same applies to various other people you listed as well, like Bonanza/Lorne Greene, and Al Waxman/Cagney & Lacey
X Files: William Bruce Davis (the cigarette smoking man)
The New Avengers: Linda Thorson (Tara)
Hill Street Blues: Det. Mimi Kuzyk
(edit: duh; Mimi Kuzyk playing Det. Patsy Mayo)
Murphy Brown: Colleen Dewhurst (Avery, Murphy's mother)
Cheers: didn't have a Canadian actor, but it made up for it by having an archetypally Canadian character ;) -- and John Ratzenberger (Cliff Claven) played the stupid Cdn officer in the Brit-Belgian WWII series Secret Army ...
Grey's Anatomy: Sandra Oh
CSI and its offspring don't appear to have Cdn leads ... and, well, just look at the results. Ew.
NYPD Blue: Gordon Clapp (Det. Greg Medavoy) -- not a Canadian, but also started his career here (won a Gemini on Street Legal), and one of his first film roles was in John Sayles' Return of the Secaucus Seven, which, while not Canadian, is kinda like a Cdn version of The Big Chill. ;)
ER: Gloria Reubens (Jeannie Boulet, physician's assistant, no longer there)
http://www.answers.com/topic/gloria-reuben"She states that she is Jamaican Canadian, and not African-American. ... In 1996 she was chosen by People magazine as one of the 50 most beautiful people in the world."
Grace Under Fire (hey, it ran for 5 years): Dave Thomas (Russell Norton, pharmacist)
And of course, to scrape the bottom of the barrel ... and demonstrate that having a Cdn lead isn't exactly a guarantee of
quality:
90210: Jason Priestly
Friends: Matthew Perry
Party of Five: Neve Campbell
The O.C.: Kelly Rowan
Lots of other series make liberal use of Cdns for guest-star and character roles.
Andrea Martin: not a lead in prime-time series in the US, but Quark's mother in DS9. Star Treks were full of us. (As was SNL -- add Norm McDonald to the list.) Graham Greene was there and in others like Northern Exposure, as a recurring though not lead character.
... Oh look:
http://ccr.ptbcanadian.com/star-trek/Cdn Star Trek connections. With a bunch of others frequently seen on the US airwaves, even if not as leads. (Saul Rubinek, for example. David Steinberg.) But huh, I hadn't realized that Martin was actually born and reared in Portland, Maine, although she started her career in Toronto.
Maury Chaykin: another one who's from the US, but who does so much work in Canada, in such Canadian stuff, most people think he's Canadian ... but aha, now I get it; his mother's Canadian.
If we broadened the theory to say simply that a US series can't succeed without a
non-USAmerican lead of some sort, then we can include House, my new favourite purely because of Hugh Laurie, who's almost as cute as Bruce Greenwood and Gul Dukat.
http://news.bbc.co.uk/1/hi/entertainment/tv_and_radio/4683193.stm(Anybody else having the great pleasure of watching Blackadders for the first time on BBC/BBCKids? For some reason, I never did before.)
Now, if we were to start listing all the cities (and other geographic features) played by Canadians on US TV ... !