Going along with the “you can say absolutely anything as long as it’s in quotes” mantra:
the stronger the insult the better
I’m not 100% versed of insults used by English-speaking countries other than the U.S., but the “stronger” titles for people who are a) incompetent, b) lacking knowledge, or c) ignorant oftentimes use the word “fuck” as an adverb (a word that should be avoided in most conversations due to its stigma), such as:
1) “idiot” -> “fucking idiot”
“Idiot” being a noun for somebody who is incompetent or not knowledgeable, as would “fool”
2) “retard” -> “fucking retard” -> “retarded fuck”
“retard” being a noun for somebody who is either physically or mentally handicapped, colloquially used as a euphemism for somebody who is remarkably a) unintelligent, b) incompetent, c) socially awkward. The socially awkward description stems from the fact that there are types of mental disabilities that could make one not as socially capable as a person without said disability.
“retarded fuck” meaning the insulted is a “fuck”, which usually doesn’t mean much in and of itself, but detailed with “retarded”. Keep in note that all of these are HIGHLY subjective, and insults are very dialectic. However, if you come to California, this is what you’ll probably hear.
3) “dumbass” -> “fucking dumbass”
“dumbass” being a compounding of “dumb” (similar to foolish) and “ass”, a euphemism for one’s rear or a donkey that has been tacked into the expression for good measure.
4) “dumbfuck” -> NOT “fucking dumbfuck”
“dumbass” being very similar to “dumbass”, with “dumb” meaning foolish, and a curse added on to the end. Do not use the adverb “fucking” before this word because it has already been added to the word itself. I consider this word to be one of the most concise insults of this list, along with “idiot”.
While those are the “strong”est I could think of, I do caution you to only use these with very close friends who are 100% casual with cursing and insulting like this. Misuse of a language is fast road to a hospital, and I implore you to use these with high discretion.
For things such as work encounters with coworkers, I would recommend some of the words you listed, such as “silly”:
A: How do put this square peg in the round hole?
B: You don’t put the square peg in the round hole, you put the round peg in the round hole and the square peg in the square hole, you silly.
Again, these expressions are highly dialectal, so you’ll hear different words or expressions in different cities and countries.