Julian, as usual a well thought-out post...I thought you were just going to compare Supes to his more obvious imitators.
For me, the character that is the MOST like Superman is, strangely enough, Spider-Man, for the simple reason that Spider-Man is the character that has had so much of who they are built into the idea of the secret identity. Spider-Man's secret identity is the source of all the irony, conflict and humor in his book: whether it's Doc Ock secretly conspiring to marry Aunt May, or the irony of Parker selling Spider-Man pictures to a publisher with an anti-Spider-Man grudge, or Flash Thompson thinking Peter Parker is a wimp, but being Spidey's biggest booster.
Well, certainly there are the superficial similarities...nebbishy alter-ego who can't catch a break leads a secret life of adventure using hidden powers. It's probably also no coincidence that Peter Parker, as originally envisioned, wore glasses and worked for a great metropolitan newspaper.
Beyond that, though, I don't know. It's true both characters have a lot tied up in their secret identities, but it always seemed more like a game with Superman, and deadly serious with Spidey. I never had any doubt Clark/Superman could fool Lois for the millionth time, and if all else failed he could take her to the fortress and hit her with a ray of forgetfulness, or something. (Thus my powerful aversion to "secret identity" stories, in my opinion easily the least interesting aspect of the super-mythos and a near-criminal waste of story potential). On the other hand, with Peter the stakes are real and huge...lives hang on whether his ID is kept and in the end he's just a working-class kid with no ability to undo reality or reverse time...once his secret's out, it's out.
For me, Superman (or Kal-El, anyway) will always be the "real" person and Clark Kent merely a disguise, while Spidey is Peter first and foremost. Spidey's "elan," as you put it, his swashbuckling, wise-cracking dynamism is a release for Peter Parker, the introverted, brooding nebbish. Spidey is an outlet for Peter's id, whereas Clark Kent is more of a retreat for Superman, the equivalent of the "Garage" sign on a taxi-cab. Yes, he's in motion, but he's in his down time.
If you're going to grade everything based on the importance of secret ID's, then I'd say Spider-Man is closer to Batman. In both cases, the costumed identity provides an outlet for the wearer to work out some inner demons, to cut loose and experience adventure in a life that's otherwise dark and unfulfilling.
For me, the Marvel character closest to Superman would have to be Captain America. Both are men who are more than men...they're icons and symbols of something greater, a fact they're aware of and so do everything in their power to maintain the power and value of their symbolism. Both derive a sort of extra "super power" from this symbolism that sets them above their costumed peers and makes them the assumed leaders in any superhero gathering. And both have done a pretty good job of symbolizing the American zeitgeist at any given time, representing the will of the people more than allegiance to any particular administration.
I suppose you'd lump Cap in the same category with Green Lantern, though, in that who he is without the mask is inconsequential and almost irrelevant (thus Steve Rogers has a spotty, jumbled resume to rival Hal Jordan's). But I still think the analogy to Superman holds, in that Clark Kent isn't necessarily that complex or nuanced a character. He is the prototype nerd and wimp, not much more than a pair of glasses...a cypher. Whereas Peter Parker is a fully formed human being, and who he is affects everything in Spider-Man's life.
That's also why Doc Savage just as a character was never that interesting: he was the perfect paragon Doc Savage 24/7, so of course he'd be a very alien, unemotional person.
Well I think in the Doc novels some effort was made to keep him remote and unknowable. Sometimes in the early books he didn't even show up all that much...the character interaction and byplay was left to his assistants and he showed up from nowhere to save their bacon. I think if the narrative probed deep into his thoughts, he'd have lost the sense of mystery and awe. If Dent had gone in another direction, Doc could have been a fascinating psychological study even without a secret ID...I mean, how screwed up would a guy be who spent his entire childhood on a desert island being tutored by scientists instead of playing with other kids?
Also, don't forget that the Fantastic Four never had secret IDs and a lot of people consider them as interesting and well-developed a set of characters as comics ever produced.
The character most UNLIKE Superman is, oddly, Doctor Strange, for several reasons. The first is that while Superman is the point of origin of the various things that go into what a superhero is, more than any other Marvel or DC character, Doctor Strange couldn't be LESS of a superhero: he's only a superhero because of his proximity to the Marvel Universe. Any other context, he'd be a fantasy or adventure character.
Interesting to note that I like Dr Strange better than any Marvel character, and probably for a lot of the reasons you mention. He's not only free of the usual sock-bam-pow plotlines that define superhero comics, not to mention the tiresome burden of a secret ID, he also is an awkward fit in the Marvelverse and thus for the most part unhindered by the enormous amount of continuity baggage that ties every other Marvel superhero to his brethren.
I like your description of Strange as a truth-seeker on a spiritual journey. Of course he starts his life journey as the most ignorant and self-absorbed being possible, and from that point on his entire career is about becoming something greater...expanding his mind and exploring possibilities. The most important thing he knows is that he doesn't know much. That's a maturity most superheroes never attain.
Anyway, if I were to pick the hero most unlike Superman I suppose it would be Wolverine, a guy who's all about giving into urges and cutting loose, the opposite of Superman, who's all about restraint and responsible behavior. I would have picked the Punisher, but it's impossible for me to define him as a "hero" no matter how you twist the word.