Most popular in the public eye? Batman, though he's just as unrealistic as Superman.
Seriously? A multi-billionaire, with unlimited resources, who comes off as a total player in his "public" life, has women throwing themselves at him left and right, in and out of costume; is a genius lvl 1,000 ninja, scientist, engineer, mechanic, et cetera without super speed, or being 80+ years-old for all that study. As opposed to an alien who is a genius with super speed, with which he can have projects in his Fortress of Solitude, thus having a better excuse as to why he's a scientist/engineer/etc.; but, was raised on a simple farm; works at a newspaper; is in love with, and stays faithful to, one, head-strong, mortal woman, though there may be other, super, women who want him; he's a nice, decent guy; doesn't want to take over the world, just wants to live a regular life, but has a sense of duty to use his powers for good.
Batman is who they relate to... no, that's wrong. Batman isn't the one they relate to, he's a guy's wet dream. Women throwing themselves all over him, in and out of the mask; tons of money with which he can do whatever he wants; power; glamorous danger as an ass-kicker; a genius in uncountable areas of expertise even though he's only, at oldest, usually, in his thirties? Next to all that, who would want to be the "boring" Superman, who is an alien who dreams of nothing more than a simple, happy, life with his willful, adoring wife who doesn't let him get away with any shenanigans?
My favorite? Superman. Batman's alright, but I find the capacity for a healthy relationship and fidelity sexier.
Though the new Superman movie is coming out and I'm actually really looking forward to it \o/ but right now Batman is rolling in its popularity with its current movie that came out and the new rumored video game \o/
If you mean to the general public, then Bats, no question. He's just had a crazy popular trio of movies by a crazy popular director.
If you mean to the comic book audience... still Bats, just go count how many Bat-books are currently being pushed out by DC.
The money talks and it says "Batman".
However I have tentative hopes that Man of Steel will give Supes a much needed shot in the arm and return him to the public eye with the same awesomeness that the Dark Knight trilogy did for the Bat (and Superman Returns failed utterly to do).
I really want to see people get excited about Superman again... it's been a long time.
Personally I have a massive amount of Superman/Batman crossover books, I like them a lot as a double team and couldn't pick a fave. When Supes is well written (like in All Star) he is awesome and likewise when Bats is poorly written (like in his All Star, funnily enough)) he sucks like a Dyson.