DXM is a lot less damaging than PCP, but since its in the same drug class there is always the whole "Olney's Lesions" Theory to think about. I've never done meth, mdma or pcp, the worst effects I've got from a drug were from salvia (lungs were filled with tarrish shit for weeks, and I was having occasional moments for days after use where I would start to slur my words and feel brainless for a few seconds.) and Nitrous (Much worse lung problems than salvia weed or cigs, and some temporary brain damage that lasted maybe 5-6 months after quitting, such as lack of coordination and concentration, feeling slightly disorientated from time to time.) Its funny that the safer drugs when it comes to negative effects on the brain are the Best drugs, Opiates and Benzos, and of course some others that are riskier such as weed and mushrooms are still not very bad unless your eating shrooms every week or something. Either way, all drugs effect the body and mind in different ways, and to a certain point, in different doses, so It is really hard to say which drug is the worse all together.