I'm almost certain I shouldn't admit this, but... my first idol was Micky Dolenz of the Monkees. Yep, when all the girls were going googly over little Davy Jones, I preferred Micky.
Next up was Laurence Olivier. He's the only celebrity I wrote to from whom I got an answer back - oh, not from him himself, of course, from his private secretary, but it was on his stationery, with (I think) his home address on it! Very exciting.
Jeremy Brett (known this side of the ocean for his role as Sherlock Holmes) - that was mostly because I was a big fan of Sherlock Holmes, and was absolutely gobsmacked that they found an actor that looked EXACTLY the way I'd pictured Holmes in my head.
Sean Connery, in any role EXCEPT James Bond (don't ask me why, except I just don't like the James Bond films, regardless of who plays him).
Maggie Smith - I adore her, I think she's a marvellous actress. In fact, I adored her so much that I even managed to talk my parents into letting me go on an unofficial school trip to Stratford (Ontario) to see her in The Way of the World (and it wasn't until I found the programme for it years later that I discovered the actor playing opposite her was - Jeremy Brett! Arrgggh! But that was before Sherlock Holmes.). (It was an unofficial school trip because we went in cars rather than on a bus, and we went because a lad from our school was a bit player in the play.)
More recently, I took a fancy to Ewan McGregor, who has to have the goofiest, most adorable grin on the face of the earth, not to mention a fine set of family jewels which appear in almost every film he's ever been in except Star Wars (family fare, and all that).... ahem. (TD fans herself)