Uh let me think... we have to separate between the real drugs (pure substances) and the cut shit that is commonly sold and taken.
Pure and well synthed coke, amphetamine, heroin, ketamine, etc. all are more or less easy to snort and don't cause that much burn or even damage.
It's really about the impurities, cuts and all that.
2-fluoroamphetamine and 4-FA burned pretty bad (but that was probably also due to the synth partially). Also some of these cocaine imitates they sell or have sold in the UK, I've just tried two or so due to curiosity and they were shitty - I could easily imagine much better ways to legally imitate cocaine, but well.. they seem to lack the creativity or are just about the quick money because their shit seems to sell well enough, I can't believe why but it is a hard fact.
AH-7921 is quite caustic by itself, so this might be the best candidate.
About the most disgusting substance ever - probably
zopiclone. This stuff is so horribly bitter that I'd probably identify it out just a 25mcg sample or so. You'll have a metallic taste in the mouth the day after if you've taken more than one pill.
The
solvent DMSO which is used to make non-bioavailable substances working like SARMs etc. has a pretty ugly smell and taste too.
Peace
