I'll slightly disagree with thomas' suggestions...
The top three IMO would be diacetyl, astringent, and DMS if I had to delve further I'd go for vegetal and medicinal
Astringency is where I seem far too many people go wrong. It is not a flavor or an aroma, it is a sensation you sense, you feel. Getting people to wrap their brain around that and stop saying something is astringent when it isn't can be tough.
The reason I would not do acetaldehyde is you don't find it that often in beers, perhaps every other comp at worst. Phenolic can be spice derived, yeast derived, smoky, plasticy, etc and sometimes phenols are excellent, sometimes faults. It isn't one you want to hit and run, it is almost a sensory topic unto itself.