On the occasion of the handoff from William Petersen to Laurence Fishburne on “C.S.I.: Crime Scene Investigation,” beginning tonight.

Worst things first:

5. Valerie Malone (Tiffani-Amber Thiessen) replaces Brenda Walsh (Shannen Doherty), “Beverly Hills, 90210,” 1994. “Omigod, have you heard about the new girl at school? You know, the one staying in Brenda’s room? She smokes pot! And she sleeps with boys! And she doesn’t do her homework!” Whatever. Look honey, you want to replace my beloved Brenda? Kill yourself and hope you get reincarnated as Shannen, because that’s the only way it’s gonna happen.

4. Cousin Cindy Snow (Jenilee Harrison) replaces Chrissy Snow (Suzanne Somers), “Three’s Company,” 1981. Just as all cute rednecks aren’t the same (see No. 1), neither are all simple-minded blondes. I know what you’re saying: Chrissy was dim, Cindy was klutzy. Totally different, right? Wrong. If you think that was the only distinction between these two, you weren’t paying attention.

3. Cousin C.J. Barnes (David Spade) replaces Paul Hennessy (John Ritter), “8 Simple Rules … for Dating My Teenage Daughter,” 2003. I never watched a single episode of this show, but even I know you don’t replace John Ritter with David Spade.

2. Jack Ewing (Dack Rambo) replaces Bobby Ewing (Patrick Duffy), “Dallas,” 1985. When Duffy announced plans to quit the show, producers quickly brought Rambo aboard as the Ewings’ long-lost cousin. Like Bobby, Jack wasn’t afraid to stand up to resident bad guy J.R. (Larry Hagman), but Hagman and Rambo lacked chemistry, severely undermining the show’s Cain-and-Abel dynamic. Rambo stuck around for awhile after Duffy returned, but what was the point? To paraphrase another famous Texan: “I knew Bobby Ewing. Bobby Ewing was a friend of mine. You’re no Bobby Ewing.” A footnote: Rambo, a closeted homosexual, died of AIDS in 1994. Rumor has it he was subject to much homophobia on the “Dallas” set. He was a fine actor (and possessed possibly the most obscenely large bulge in prime time history). Too bad he was given the unenviable task of replacing Duffy.

coy-and-vance-duke11. Coy and Vance Duke (Byron Cherry and Christopher Mayer) replace Bo and Luke Duke (John Schneider and Tom Wopat), “The Dukes of Hazzard,” 1982. It turns out countrified pretty boys with feathered bangs and tight jeans aren’t interchangeable. Who knew?

And now the best:

5. Aunt Sandy (Sandy Duncan) replaces Valerie (Valerie Harper), “Valerie,” 1987. Harper famously quit her self-titled sitcom in a contract dispute after production wrapped on its second season. When the third season began in September 1987, it sported a new title – “Valerie’s Family – and a new lead actress: Aunt Sandy, who moved in to help the family deal with Valerie’s off-camera death. (At the time, TV Guide reported producers considered renaming the show “Who’s Valerie?”) Eventually, “Valerie’s Family” became “The Hogan Family” and moved to CBS before completing its six-season run in 1991, but it didn’t earn the top spot on this list because of its resilience. Instead, it’s here because it was a lame show, and Harper, one of the great sitcom stars of the ’70s deserved a better material than what she was given here. She was smart to leave when she did.

4. Rebecca Howe (Kirstie Alley) replaces Diane Chambers (Shelley Long), “Cheers,” 1987. I’ll always prefer Diane, but Rebecca wasn’t a bad replacement. Alley exhibited admirable fearlessness in the role, although her performance occasionally ventured into shameless scenery chomping.

3. Stephanie Vanderkellen (Julia Duffy) replaces Leslie Vanderkellen (Jennifer Holmes), “Newhart,” 1983. Hey, “Three’s Company” producers: This is how you replace one cousin with another. Spoiled Stephanie was infinitely more interesting than dull Leslie and one of my all-time favorite sitcom characters.

the-west-wing32. Rep. Matt Santos (Jimmy Smits) and Sen. Arnold Vinick (Alan Alda) replace President Josiah Bartlet (Martin Sheen), “The West Wing,” 2005. OK, this is kinda cheating. Sheen never really left the show, but in the final season, Bartlet appeared much less frequently as attention shifted to the race to succeed him. I am the first to admit that “The West Wing” suffered whenever Sheen wasn’t on screen, but credit the producers with crafting two compelling characters – Santos and Vinick – that almost filled the void created by his absence. I’m particularly fond of the Vinick character; why can’t Republicans like this exist in the real world?

1. Jon Stewart replaces Craig Kilborn, “The Daily Show,” 1999. OK, another cheat; this isn’t a scripted show. But are you going to argue with it?