Why's definitely the one of choice... why? <-- as yeu can see, yeu can't even ask why yeu chose that answer without using why XD Well, I suppose yeu could say "whot's yeur reasoning behind such?" but honestly, that's just a roundabout way of asking why.
So whot's the point of the others? Let's see!
Who - Only gives specific detail, it's not particularly useful most of the time, since usually WHO did something isn't nearly as important as WHOT they did or WHY they did it. As such, who is rarely relevant, and is usually of little more use than for gossip.
What - So whot happened? Well it's important to know the specifics of WHOT occurred, but generally, the reasoning behind why it occurred is more relevant to recreating, or preventing reoccurrance. Still, if yeu don't know whot the situation is, it's not that useful either; what is a very useful question.
Where - Once again, similar to who, limited information is contained within where. It tells yeu where something occurred, but that's it. It doesn't give much useful information, short of the location. Sometimes, a location is highly valuable, but without other information, it's generally of little use. Just saying "Queens street" doesn't tell yeu much of anything. It doesn't tell yeu whot's happening, who'll be there, why it's important, how it's relevant, all it says is a location. If yeu know OTHER information, then WHERE can be useful, such as if yeu know there's a terrorist attack coming soon, but yeu don't know WHERE, that's a problem. But without knowing of the attack, just being told 'queens street' doesn't really provide much of value.
When - Timing is everything, but then again, so is location. When's pretty much the same as where. It doesn't give much more than one specific detail, which if yeu have all the other details, can mean everything, but without the rest, it really doesn't do much to help. If the CIA were told 9/11 with NO other information... they couldn't've prevented the WTC bombing. So they have a date... and an entire planet of people, with a ridiculous number of possibilities of whot could occur on that date. Not useful in and of itself.
How - If yeu know HOW something occurred, yeu can recreate the events which led up to it, or at least understand it better. Knowing how something works, however, is of limited value. It's nice to be able to recreate it! But... yeu can't improve upon it. An example of this actually would come from an anime - robotech. The Zentradi knew HOW their warships worked... they knew how to fix them, they knew how to use them... they didn't know WHY they worked though. They understood that if yeu put a cable from the generator to the weapon, and pull the trigger, it fires. They didn't understand the WHY behind why any of this was necessary, which prevented them from ever improving upon the designs, or removing themselves from slavery to the robotech masters. They couldn't make new technology for themselves, because they didn't understand why the stuff did whot it did, they just knew that it did it.
Why - Is the single most broad reaching question of all of them on this list. Why did yeu do that? Why does it work? Why didn't yeu do something else? And of course... just "why?". Adults often get pissed off when children ask why... it's a "childs" question many believe. Hardly true, kids just want to understand their world. Adults have often been jaded into believing they don't need to understand, they just need to know whot works, where to be, when to be there, who to meet, and how to do their job. The reasoning behind why any of it has to be that way requires thought, which's something that's been forced out of us by our society. We are herded into avoiding asking "why", because "why" is not knowledge... it's understanding. And understanding is true power. If yeu know WHY everything occurs, yeu can use it to yeur advantage. A nice example here is of the cargo cultists... there's been a few of these pop up over time, but the first ones known were when primitive island dwellers were subject to their islands being used as air bases for american and japanese troops. They knew who these people were, they knew why they were there (there was a big conflict going on; specifically WW2. They didn't know the specifics but they knew there was a war). They knew where, why, when... they even knew how... and that was the problem. They knew that for planes to drop supplies, that they would need an air strip, and an air control tower, they'd need people to talk into the headsets... they knew how it worked by observation. The problem is... they didn't understand WHY any of this existed. Why do yeu need an airstrip or tower? No clue! It must be something of an idol to the gods! ...Sadly, this meant that they would carve headsets and build control towers out of wood that looked the part, but they didn't know why they existed so the matter of 'how' was bastardized into useless information. They would talk for years on end into their little wooden radios, but never receive an answer, nor a shipment of cargo. They knew how to do the job, but because they didn't know why, they didn't understand their mockups weren't good enough.
Why is the power of comprehension, and the ability to do more than merely 'know'. Knowledge is useful, to a degree. Understanding lets yeu use that knowledge to its' full potential benefit. Yeu can 'know' everything, but still be stupid. A dictionary KNOWS all the words... but doesn't understand why, so can't put them into coherent language. Why is the hardest question of all to answer, and it contains within it the most powerful information of all.
That's why I chose why. Because it is the key to not just knowing, but to understanding.
It doesn't matter if they're right. If they can't proove they're right, then they're wrong. No matter how right they may be.