46 hours sober

[Altruist]

If by "hardest to smell" you mean after consuming a moderate amount, there is no alcoholic beverage that reliably avoids detection on the breath.
A common misconception is that clear spirits such as Vodka are nearly odorless. They may have less of a distinctive aroma than beverages such as Whisky, Wine, or Beer, but alcohol itself produces a characteristic odor that can still be noticeable.
A few points:

• Dark spirits, wine, and beer often leave more recognizable aromas because of congeners, fermentation byproducts, and flavor compounds.
  
• Vodka is often perceived as the least distinctive-smelling alcoholic drink, but people can still detect alcohol on the breath after drinking it.
  
• The smell of alcohol comes not only from residue in the mouth but also from alcohol being exhaled from the lungs after it enters the bloodstream. Because of that, mints, coffee, gum, or brushing teeth do not reliably eliminate the odor.
  
• Individual factors (amount consumed, body size, metabolism, time elapsed, and the sensitivity of the observer) matter more than the specific beverage once alcohol is in the bloodstream.
  

So, if the question is purely comparative: vodka is often considered the least distinctive-smelling alcoholic beverage, but no alcoholic drink is truly difficult to detect on the breath after consumption.