Penny, from the Inspector Gadget cartoon. Not nearly as obscure as some of those discussed above, but it’s the best I’ve got.
Jane Pauley: she wasn’t obscure but you’ll have to admit she’s an unlikely TV crush for a 12 year old.
Also, Maren Jensen–Athena on the original “Battlestar Galactica.” Only reason I ever watched that show.
When I was 8 years old, Tom Jones made my tummy feel all wiggly inside. On a black-and-white 13" TV, he jump-started my awareness of what the wear tight pants and shake your booty thing was all about.
Not obscure, but an adolescent movie crush that has endured: I’ve had the incandescent hots for Anthony Hopkins for over 30 years.
Have the lambs stopped screaming yet?
I always liked older men, even as a kid, so here are some of the actors I crushed hard on:
Monte Markham
Ed Nelson
Lloyd Haines
Bo Hopkins
J.D. Cannon
I also had a big old girl crush on Carol Lynley. Still do.
Talk Lecter to me, darlin’.
Keep that up and I’ll invite you to tour my memory palace. 
My first TV crush was for Angela Cartwright on the Danny Thomas Show. She was just a few years older than me and I thought that she was the prettiest little girl in the world.
Later when she starred in Lost in Space my adolescent attention had turned to the big girls like Honey West so her Penny character didn’t pull on my heart so hard.
I can’t remember the actress or TV program, but I do remember a crush I had on Rhoda the Robot. Anybody know the name of that actress and show?
Or Lani O’Grady.
::drools::
You know, furthermore, that Jo had equal appeal to us het boys because of her silky raven tresses, sensuous full mouth and in-you-face Irish-Italian intensity. 
My first hint that maybe there was something to this whole “Girl” thing was from Elektra Woman and Dyna-Girl. Specifically the scene where they’re being “stretched” by a cheesy special effect.
Now that you mention the Dyna-Girl thing, I had a girl crush on Judy Strangis too. Lordy, was she sexy with those pony tails on Room 222.
I can see that. She was such a badass. A gorgeous badass. I wanted her to wreck me.
Brigittaaaaaaaahhhhh
She has the cool hat, she sings a little solo in the lonely goathearder, she read books, and she was clearly daddy’s favorite.
Jenny Agutter, maybe not obscure by SDMB standards, but probably moreso by general standards.
Greta Scacchi from The Coca-Cola Kid.
What would you ask for a girl and goatherd? Ladyhodeladyhodelay! Ho!
I vastly preferred Lynda Carter/Wonder Woman to this show (Elektra Woman and Dyna-Girl), but I gotta admit they were quite easy on the eyes, especially Diedre Hall! Yummy!
I certainly had a crush on Mrs. Peel, and found I still do, when I stumbled across an episode a few years back.
My first TV crush, though, was on Annette of the Mickey Mouse Club. She was the only one that had tits. (At least she was starting to have them.)
Brandon Cruz The Courtship of Eddie’s Father
Peter Brady
Danny, not Keith
Mickey, Mike, and Peter (in that order), but not Davy
Horschack
Pete Dixon, ever so compassionate teacher on Room 222 (btw - did you know Mark Hamill was in two episodes of that show?)
eta: Det. Sgt. Ed Brown, Mr Ironside’s right hand man
…VCNJ, are you me?!?
…and add Dave Seville, from The Chipmunks. Now **that’s ** obscure.
The first TV crush that I recall was for Lisa Gerritson who played the daughter on “My World and Welcome To It,” an obscure show based on the work of James Thurber.