Possibly even earlier, Tim Considine as Spin in the Spin and Marty episodes I saw on the Mickey Mouse Club.
Marcia Brady
Catwoman (Julie Newmar version)
99 on Get Smart
Truly Scrumptious
Tevye’s oldest daughters when they’re leaning out the window
Maria Von Trapp
Oldest Von Trapp girl (Charmaine something)
Ginger
Several it appears
Maybe it was the sly, mysterious smile, as if she were about to do something naughty, which all of us boys in that era wished she would? 
Kristy McNichol as Buddy in Family.
Oh, yeah. I’ve been nursing a crush on her for almost my whole life. Odd thing, though. I know I remember watching the show as a child. My mother was into stuff like that–James Bond, Get Smart, etc.–but a Google search, just out of curiosity, tells me The Avengers first aired in the US at 10:00 p.m. on Mondays. I wouldn’t have been allowed to stay up till 11:00 at that age. I wonder if there might have been subsequent airings at an earlier time slot.
Sheena, played by Irish McCalla
Miss Crabtree, played by June Marlow
Sigh!
Another vote for Joy on the Bugaloos.
I think it was probably Meredith Baxter on Bridget Loves Bernie.
George from The Famous Five, I think.
THAT’S the show I vaguely remembered from childhood! They’e on Diz+ now!
Me? Emma Peel, of course.
Pepe LePhew
Seconded
Ursula Andress in Dr. No (1962) and Casino Royale (1967) and a little bit later in a scholarly Playboy article on The Loves and Times of Scaramouche (1974).
And Julie Newmar as Catgirl.
Apparently, she didn’t do it for me. Those Batgirl episodes were the best.
Yvonne Craig, also green alien in Star Trek.
Thirded. (Though perhaps equally as enamored of “Samantha Stevens” on Bewitched.)
That would be mine as well. She was so wonderfully cool.
Probably Thalia (Tuesday Weld) on Dobie Gillis or Emma Peel (Diana Rigg) on The Avengers.