I'm fully in agreement with Zonker on this one... I was going to suggest Superman first, Batman second. My reasoning is that you're planning to hold the book(s) for what sounds like 30-50 years (without knowing your age)...
And with that type of long-term plan, it seems to me that you want to absolutely stick with the iconic aspect that Zonker mentions.
If you had to bet on Golden Age characters who'll *definitely still be around in 30-50 years, who would you put your money on?
Beyond Supes, Batman, and a couple other heros (Wonder Woman, Green Lantern, Flash), the only others I can think of are Mickey Mouse and Donald Duck, since it's safe to say that Disney will never let those characters leave the limelight.
But funny animal stuff, much as I love it personally, doesn't have the broad fan base today that will ensure significant appreciation over decades, at least not to the extent that the superhero characters will.
World's Finest seems like a good choice, along with the eponymous titles for Supes and Batman, and of course Detective and Action...
Good luck!