Oh boy, I guess I'll be first! I have color-treated hair, so this influences this list, I believe. I've learned not to stray too far with hair products, as I've found just a few that are cheap and I like.

Here's my list of worst:

Mane and Tail Conditioner for me too - for the same reason. Meanwhile a girlfriend of mine swore by this as the HG and be-all of all hair conditioners. Total waste on money. Maybe this works on people who have VERY coarse hair, but not for anyone else.
Aubrey Organics JAY Shampoo(really wanted to like this as I was trying to go all natural, but it left my hair dry, coated, greasy at the same time, and I have dry hair to start with, plus my hair just didn't feel clean to boot!)
Aubrey Organics GPB Conditioner(similar complaints, dulled my hair, plus it didn't really condition either)
Avalon Organics Botanicals Lavender Shampoo and Conditioner - used to like this several years ago, but they CHANGED the formula-UGH! Left my scalp an itchy burn-y mess!
Any KMS product for curly hair - whole line stunk IMO. Left my hair feeling coated and dull, and limp, to boot.
Most beauty salon shampoos(with only one exception) that the cutters shampoo my hair with - they all leave my hair not feeling clean, likely because they only lather up once, whereas I lather up twice(well, I only shampoo once or maybe twice a week).
health-conscious, visionary artist in transition