Being limited to one roa or to one class of drugs? It would suck having to smoke or plug everything, but it would also suck bring limited to one type of fun.
MDA (because isn't mescaline active from 300mg approx or is 300mg an average dose?) in which case i would go for the low dose mescaline (hard to come by)
3mg flunitrazepam during your math finals or 500mg of ketamine during your sports finals (at uni for example)
I'd prefer neither, but if I had to, white russian probably
A shot of 90+ % liqour or some really weird foreign liquor ( I.e i've heard of some places having artichoke liqour that tastes absolutely terrible apparently)
I haven't done ketamine before, but I have a strict rule to myself about staying away from e and speed, but I always enjoy trying new substances so i'd have to say ketamine