Amphetamines are easily what I have the most negative reaction to. Makes me feel terrible, makes me crave other drugs, leaves me feeling cracked out and sleepless for 20+ hours once the rush is over, makes eating even harder than it normally is (used to have an eating disorder in high school), the crash is horrible... hate em.
As far as just useless and non-recreational goes, cyclobenzaprine is up there. Sitting around for hours waiting for some faint lacy visuals and an uncomfortable antihistamine-like nervous energy. Bleh. If you use them for anything, use them to potentiate opiates, don't expect to get high on them, and if your doctor wants to give them to you, ask for a benzodiazapene muscle relaxant instead, since those are far safer and far more fun and controllable.