Judi Bowker from Clash of the Titans
Ryan Lambert, the bad kid from The Monster Squad, which will be celebrating its 20th anniversary with a special two-disc DVD release. I had a poster of him in my locker and everything.
Another vote for Randy Mantooth on Emergency!
Larry Wilcox on CHIPs (forget Erik Estrada; blech)
Paul Michael Glaser as Starsky
My really older man crush, Jim Hutton , when he was starring as Ellery Queen. I also crushed on Tim Hutton, when he was in Ordinary People, and then again, as Archie Goodwin on Nero Wolfe (of course, I have a massive crush on Archie Goodwin himself). But I crushed more on Jim. Man, I loved that show.
Alejandro Rey from The Flying Nun was very hot. (Man it’s hard to find a picture of him where he isn’t completely hidden from view by that skinny nun.)
She may have been hot (OK, she WAS hot), but she was the worst actor in that film, and that includes the ridiculous Harryhausen monsters. 
Those monsters weren’t ridiculous! They were awesome.
About the same age as I was then, and I don’t know why, I had a crush Glenn Scarpelli from One Day at a Time. And look how he turned out!
My pre-teen crushes:
Jennifer of the Jungle from Electric Company
Martika from Kids Incorporated
Princess Ardala from Buck Rogers
Dyan Cannon (maybe not so obscure, but in her prime, she was hot)
Kathy Coleman from Land of the Lost…
Oh hell yes!
Oh stop it. Don’t even make me release the Kraken on you.
Deanna Lund from Land of the Giants.
I just remembered one that really was from my youth (as opposed to “Blake’s 7” which was during my second childhood) : George Maharis from the TV show “Route 66”. Ooh La La!
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If we’re going to toss in music personalities (if that’s OK with the OP) then I should probably add Annabella Lwin and Kim Gordon…
She was also in the series Black Beauty, which used to play on Nickelodeon in the early 80s. Mmmm. It was all the sweeter because I had already seen her butt.
Obscure? Ok, I’ll go with Olivia Barash, the victim of the evil clown rapist in yet another traumatizing episode of Little House on the Prairie.
Bernadette Peters from Silent Movie
That one and The Jerk.
I’ll be in my bunk.
No, Kristy wasn’t one of the Bradford girls. You’re probably thinking of Family.
Acting? Monsters? Bowker butt!
Reminded by this thread: Tasha Yar. Hers was the only action figure I ever bought. Emma Thompson from Much Ado About Nothing was another huge crush of mine.
Sandy Kenyon. Is that obscure enough?