As yabob said, pure alcohol will aggressively obtain water vapor from the air. I was told, and it may be true, that a full glass of 200 proof grain alcohol will overflow until it reaches 190 proof. Does anybody here know if that’s strictly true?
Google says it destroyed too many brain cells, and peaked out at 80 proof.
So much for drinking memory!
What would 100% ethanol taste like?
From my small contact with it, not good.
No more than me. I studied chemistry at university, and in my final year we got hold of a 5-litre bottle of HPLC-grade (that’s high-performance liquid chromatography) ethanol. It was stated to be 99.8% minimum ethanol by volume, and free from benzene (thankfully!). It had to be pure as chromatography is used to separate and identify compounds. This is the stuff - Sigma Aldrich, the connoisseur’s ethanol 
Strangely, the price on that site is only about £150, or $275. I remember being told it was much more expensive than that.
We used it to make punch, and to add to lemonade etc, and we never came to any harm. We also, naturally, tried it neat.
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The answer to your question is difficult to answer, as the neat ethanol kind of evaporates as it hits your mouth. It’s difficult to swallow it without choking. The actual taste is much as you’d expect - think of strong vodka such as Finlandia (60%) and multiply it up a bit.
We never left the stuff out in a glass to test this out, I’m afraid.
Nah, I kinda understand what the point is; but I can’t believe that drinking 200 proof anything would get you drunk that much quicker than 190 proof, y’know?
Re: leaving the stuff out in a glass.
In addition to being hygroscopic, alcohol evaporates quite rapidly. I don’t know, but I suspect you would be more likely to get a glass of alcohol whose level would drop due to evaporation, and which would actually contain a few percent water from absorbing water vapor. Also note that the volume of an alcohol/water mixture is less than the sum of the volumes of the two liquids (a well-known classroom demonstration) - the absorbed water vapor may not expand the volume all that much. The humidity of the air would be a definite factor.
Well, I can tell you what Everclear tastes like. It tastes like shit. Basically like you popped open your botle of rubbing alcohol and took a swig. (Yes I know they are different types of alcohol).
In Idaho, we can’t buy it. One time a friend and I drove 30 minutes to cross the Oregon border where it can be found at any liquor store. It is great for blowing fireballs, not so great for drinking.
You are so not getting the advertising contract.
So… like Zima? 
No. Zima tastes like flat 7-Up. Everclear tastes more like flat Dr Pepper, with a dash of WD-40. 