Source: Superman's Pal, Jimmy Olsen #1 (1954)
Type: The real deal (since retconned)
Jimmy had been around since the 1940s thanks to the radio show (though you can point at an office boy in 1938's Action Comics #6 and say that's proto-Jimmy), but it's not until the 50s, and I think more pointedly, the start of his own eponymous series, that he truly becomes a comic book icon. His varied adventures between 1954 and 1969 make him much more than a teenage news photographer (Peter Parker still needs to send him a royalty check though) or the damsel in distress we see in Superman's own books. Built up as a (usually) non-costumed sidekick who calls Superman with his special Signal Watch when he's in trouble, he's allowed to become a true protagonist of the Silver Age in his own title. Superman isn't even really required.
And if I consider this to be the one true Jimmy Olsen, it's because the ghost of the Silver Age will forever hang on him. No matter how serious and reasonable the Superman mythos becomes, there will always be the temptation to turn him into Turtle Boy, or Elastic Lad, or give him some fantastic adventure, if even just for a fleeting issue. Jimmy is the original weirdness magnet - with apologies to Blue Devil - and the weirdness can never be kept at bay for long.
So in the coming year, we'll meet lots of versions of Jimmy: Transformations, alternate lives, ridiculous costumes, live action and voice actors, and many of them will be from the Silver Age. It just can't be helped. That's the real Jimmy Olsen. And these are his snapshots.