From a scientific standpoint, it's actually a bit sloppy to say that liquor is a drinkable form of alcohol. When referring to liquor, the alcohol is actually ethanol: one of the simplest and most common forms of alcohol. Pure ethanol is some pretty nasty stuff, having a flash point at 13C (55F). For comparison, diesel's flash point is around 62C (162F). You can understand why alcohol is used a propellant and fuel for vehicles.
But that bottle in the liquor cabinet is not likely to spontaneously combust if you're smoking around it, nor is it going to work as a pricy alternative to your car's gasoline. There are exceptions to that rule, and these are the liquors that could be called the Hazmat (Hazardous Materials) category.
The king, weighing in at 95% abv, is Everclear. This really isn't meant to be drank straight and is really a mixer for non-alcoholic drinks or making liquors. I keep a large bottle on hand as a food-grade disinfectant, emergency fuel source, and once used as a topical analgesic for a toothache. To give you an idea of how potent Everclear is: It takes one cup (240ml) of Everclear substituted in for a portion of the water in a standard grape Kool-Aid recipe in order to make a punch with the same average potency as wine.
Why isn't there something above 95% abv? Alcohol is hygroscopic, which is to say that at percentages above 95% it will absorb humidity from the air in order to reach equilibrium at 95% abv.
Everclear has a quaint Irish cousin, known as PoitÃn or Potcheen. Historically, it's an Irish moonshine but has recently been commercially bottled. The bottlings vary from 40% up to 90%, with the "good stuff" being well above 60%. This is actually meant to be consumed, though the most potent I've had has weighed in at 55%. It's hard to get ahold of the high-end stuff because it's so potent.
Moving into the range of stuff with flavor, high-proof rums end to float in the 70-85% range. The most famous rum in this range is Bacardi 151. If you ever see a flaming drink in a bar, this is probably the propellant used for the show. The less famous rums are commonly toted home by Caribbean cruisers, possibly to be confiscated by righteous customs and TSA agents. Remember folks: If you're going to blow a few grand to spend a week on a ship - have the forethought to know how to get your rum booty back home.
One high-proof rum of note is Austria's line of Stroh rums. I've got a bottle of the 80% stuff and it's quite nice. I especially like the pinkish tone that you'd associate with gasoline.
Whiskies have their dominant contributor to the hazmat category: barrel proof Bourbons. Buffalo Trace's George T. Stagg is the most common example of this having their 15-year old weighing in near 70% abv every year. More commonly found is Jim Beam's Booker's which is around 60-63%. This is relatively closer to the high-end of Bourbon's popular range of 43-53.5% abv.
Scotch has its own contributor to this range, but is not yet available. Bruichladdich announced they distilled a spirit at 92% abv and is to be called "X4". What its proof will be once it gets out of the cask is anybody's guess. As mentioned before, Scotch's potency diminshes with age, so a few years (three years minimum) in an oak cask is likely to do a number on the spirit. It should still weigh in well above the 70% range allowed on a plane.
The requisite warning with all of these drinks is to heed caution: they aren't your run of the mill drinks. If you drink them straight keep them limited to tastes and don't consume them like they were a shot of vodka or a Tequila chaser. But if you pause, taste, and relish in these high end proofs you'll experience both sides of the thin line separating liquor from perfume. These are aromas to be taken in by the nose... not the gullet.