this is all so very subjective, but I'll give it a go:
overrated
marijuana - a drug that makes me retarded, socially inept and tired? sign me up!

I think the drugs usefulness for psychedelic exploration/introspection are exaggerated as well...I will never understand how and why so many people want to be stoned on MJ for so much of their day...
cocaine (
crack even more so!): high is too short and not good enough to make up for the negative affects and the asshole you become while on, or in search of the drug
benzos: I think they are GREAT compliments to other drugs (especially drugs that can cause high anxiety), but by themselves they aren't that special (unless your idea of a good time is blacking out/ falling asleep).
underrated
opiates: obviously I like them very much and am quite partial, but I think opiates can be used in so many situations, and don't cause horrible side-effects or comedown...however that whole addiction thing can get ya
mushrooms: a relatively cheap psychedelic that is amazing powerful...can give great euphoria/ body high along with typical psychedelic effects...and the trip doesn't last all day!
DXM: super inexpensive, readily available, legal and the high can be very pleasurable and even introspective...I never experienced too many of the negative side effects (stomach issues, etc.) when I have used DXM, which is probably why I give it good reviews...does pack a bit of a hangover though..because of it's legality and MOA (syrups especially) people view it as a "teenager's drug", and think the high will be boring and limited...I think that this view is incorrect and the drug is underrated for that reason...