Depends on what "worst interpretation" means, plain and simple, which is context dependent. I've read the comments, and the appeal to motive is not strictly correct. The better classification is the fallacy of irrelevant conclusion. From WP:
An irrelevant conclusion, also known as ignoratio elenchi (Latin for 'ignoring refutation') or missing the point, is the informal fallacy of presenting an argument that may or may not be logically valid and sound, but (whose conclusion) fails to address the issue in question. It falls into the broad class of relevance fallacies.
As noted, it is not a strawman attack because a strawman must have a superficial resemblance lending it the appearance of similarity. Strawman attacks, which are often polemical in nature, can be downright devious in their ingenuity by attacking an argument that seems identical. The worst possible interpretation of an argument might not look anything like the original argument.
But the worst possible interpretation isn't necessarily an attack on the lines of someone's motives. What is the "worst possible interpretation"? That will always be context dependent. Accusing someone of hebephilia or pedophilia is pretty bad if the opponent is a priest or teacher. Accusing someone of being self-hating if they are a minority might be the worst under another circumstance (both types of ad hominem). Worst interpretations would seem to involve a combination of other fallacies: if your opponent says X, then you appeal to emotions, attack the origin of the argument (genetic fallacy), lay out a strawman, and cook up whatever else you think of. None of those necessarily involve imputing motivations on others, although it's so common a tactic as to be expected.
Informal fallacies are artifacts of natural language, and they do not necessarily all get named. That being said, it's very possible that someone has named it but the name has achieved little currency. Bo Bennett has an online compendium of over 300 fallacies but none seem to apply; I would also urge consideration of another point.
Informal fallacies tend to have criteria assigned to them. One of the criteria to which I adhere is one of persuasiveness. When someone violates the principle of charity so flagrantly as to present an absurd counterargument, it is questionable if the counterargument is persuasive anymore thus moving it out of of the class of informal fallacy and into some other class, like philosophical bullshit. BS or humburg as Frakfurt alternatively labels it are claims that have no regard for the truth at all and are meant to serve some other purpose other than persuasion by reason.
