Thought I'd leave this here as it's always amused me.[spoiler]Particularly the Flash's comment about Superman being just as fast but being cool about it and never bringing it up.[/spoiler]
I'd have to say Superman. I mean, let's face it, Batman's only real chance is, to have Superman slip up. Between X-ray vision and super hearing, any stealth advantage Batman may have is completely negated. On top of that, superman is fast... REALLY fast. If he doesn't want to get caught, he's not going to be, and things like laser eyes and freezing breath mean he can keep his distance. Batman's only real chance would be to gain some kind of leverage over Superman (ex: using Lois for instance), effectively using extortion and/or eventually breaking his spirit but that wouldn't be very Batman-esque of him. Honestly, the only real advantage he has is the fact that he's "the underdog" and, as such, the one the audience wants to see win.
I realize Batman has defeated Superman before, but I'm going to have to agree with [url=http://geektyrant.com/news/2013/9/3/batman-vs-superman-what-would-really-happen]this guy[/url] that those cases don't really seem to be truely definitive "defeats" per-se.
English
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One word Kryptonite
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As I pointed out though, it'd be near impossible to hide it from him unless you're hauling it around in a lead case (which is in itself a red flag) and there's the problem of delivery. Sure, Batman has a kryptonite ring, but Superman is faster than a speeding bullet. You think he can't dodge a punch? Also, which is a bigger weakness, kryptonite or being limited be the human body?