Ignore KillCops! Taking the one with guaifenesin will make you very, very nauseous!
The one you want is Robitussin Dry Cough Forte, also known as Robitussin DX. It comes in blue packaging, and the only active ingredient listed will be dextromethorphan.
Avoid the Benadryl version as it contains sorbitol, which is not pleasant.