I take no credit for this, just found it online
When people say: "Batman can kick Superman's ass" they really mean one of three things:
1. I like Batman more and think he can do anything.
2. I think Batman can win a one-sided battle where he's prepared and Superman isn't; and I've read comics like that.
3. I think Batman can beat Superman under specific circumstances, but all things being equal, Superman will almost always win.
With Case 1, there's no arguing with them. With Case 3, I think the individual is reasonable enough and there's no need to force them to say Superman will ALWAYS win unless we're suffering from a bit of Case 1 ourselves, but with a Superman bent. I think a mixture of Case 2 and a little bit of 1 is the predominant viewpoint of people who say it.
The thing is, they typically don't acknowledge the inequality in preparation or technology... it's almost always a contrived circumstance to bring about a Bat-victory by the skin of Bruce's teeth. Even if they do, they put their faith in Batman's preparation and planning (and sometimes technology) to rule the day. The following addresses that (reposted):
Despite the incredibly presistant and wide-spread belief that Batman has a dozen plans up his sleeve to take out Superman, historically it's been proven otherwise again and again. Let's take some examples:
DKR - Bats, after 20 years of planning, gets his ribs broke, his fancy suit torn to shreds, suffers a heart attack, and ends up in the ground. His entire plan relies on Superman holding back and NOT simply unplugging Bruce from his lamp-post or disabling him from space (if Supes really just wanted Bruce to turn himself in, rather than getting into a dangerous fist fight)... in other words, poor writing for Supes (out of character motivation, out of character intellect/tactics), who could have undid the Bat's plan easily.
Hush - Bats breaks his hand & resorts to thug-level hostage tactics, while admiting he doesn't stand a chance. Honestly, he just gets in one good punch (that breaks his hand) shortly after, Supes smashes through the street and effortlessly lifts a car in each hand, no worse for wear. His plan needlessly puts himself at risk and relied entirely on Clark holding back/being good, which he had no logical grounds on which the make that assumption... having a "gut feeling" that Clark can resist mind control does not constitute a good plan.
Red Son - Dies. Despite all his prep, he didn't account for Superman's allies, which he has in spades and certainly on a different power-level than- say- the Bat Family. If anything is true of Supes, it's that he has many friends that would readily risk life and limb for his sake- heck, in the regular universe, Batman considers himself one of that number... any plan meant to take Supes out has to take into account his allies, friends, family, robots, pets, etc.
Babel - Years of planning on how to stop a rogue Superman... result: an expensive synthetic rock that makes Superman MORE powerful (and doesn't even prevent him from using his powers with surgical precision). Wow. Brilliant.
Superman/Batman #2 - Owned. This is in Batman's home turf, the ideal situation to combat Superman and to enact all the theoretical plans his fans have long believed he has against a rogue Superman... but the result? Bruce isn't even Future Supes's target yet he's a breath away from death if not for a save from present-day Supes.
Lex Luthor:Man of Steel - Batman with kryptonite and prep, defeated handily by Superman. Arguably one of the most realistic portrayals of how Supes could still defeat Batman without instantly killing him regardless of Bat-prep.
Superman: King of The World - Batman with prep and kryptonite fails to stop Superman from being able to crush his throat... only with the intervention of Green Lantern and Martian Manhunter is Supes stopped. Here we see danger of Superman with limited prep, the legions of his similarly powered Superman robots.
Sacrifice - Again, Batman, king of protocols, the man entrusted with the Kryptonite ring as a symbol of trust and the express responsibility of stopping Superman should he go rogue by Superman himself... finds himself in said situation and instead of pulling out "always prepared anti-Supes plan" finds himself beaten to near death saved only by the Plot Gods.
Infinite Crisis - After the last beating, you'd think Batman would create more comprehensive plans for taking out Superman and/or Superman-class enemies. Granted Kal-L is probably levels above them, but in terms of Anti-SuperBrick Planning, once again, Bruce fails... his entire defense the Kryptonite ring. Even if it were Kal-El and not Kal-L, it's highly questionable whether the ring alone would have been able to stop a motivated rogue Superman considering Supes has wielded the ring himself when fighting other Super-persons. If Batman were truly the master of planning and prep, this certainly should have been taken into consideration.
Final score: Bat Plans Zip; Supes nine of nine.
The only cases where Batman has been able to possibly get a leg up on Supes are when he has acted like a villain and held hostages or lead an assault against an unsuspecting Supes- hardly a feat (consider: who amongst us, with Batman's fortune and training, intimate knowledge of Superman as an ally, and even express consent from said target, WOULDN'T be able to come up with an attack plan against an unsuspecting Supes?). In any case where Supes has had the opportunity to fight back he has either won or been defeated by bad writing.