Matthew's Journal

I would certainly have used "pessimal" (and its relative "pessimising", not that that has much use outside derogatory comments about crap compilers and the like), and still would regardless of OED-related objections: perhaps it's a new word or one not often heard outside technical fields, but it's consistently the word I've heard used in the context you describe, and it's instantly clear what it means to anyone who hasn't heard it before.

I can't think of any better word off the top of my head, and also support using pessimal, as it obviously conveys the meaning you want, whether it's traditionally been used to do so or not.

I guess you might also say that in my cases, the exact nuance of "worst of all possible outcomes" or "worst of all similar outcomes" might not be needed, and "worst" convey the meaning equally well. In which case, pessimal would be reasonable, but as would just using worst (or a similar word).

Conversely, you might use "negatively extremal" or something, and have the added bonus of sounding extra sarcastic.

Pessimal is the right word, use it.

I'm going to take a controversial tack and declare that there is no opposite.

Because while it's not necessarily possible to make something better (and this can be mathematically proven) I'm unaware of any case where it has been proven that a thing cannot possibly be any worse. E.g. adding to pretty much anything "and you're also on fire" would make it worse.

While we're at it, what's the opposite of 'deprecate'?

I've been using 'apprecate' for a few years, but I'm pretty sure that isn't a word.

Context: "I'd consider this interface method apprecated, but any of the others will work..."

Sometimes 'aprecate' instead.

I'd probably say "least optimal" but would pick up the meaning of "pessimal" from the context.