Usually, my goal when insulting someone is to come up with something either so filthy or comically dated that the other person has a brain meltdown and can't think of a single thing to come back with.
If someone is being particularly rude or crass, the good old, "You Sir, are a scholar and a gentleman." works pretty well.
Philistine is also a pretty good all-purpose insult. Especially when you're insulting people's choice of alcohol.
When arguing with my sister, I'll sometimes call her a jive-turkey just to throw her off her game.
And there is one thing I use only for special occasions when arguing or trading insults. This is probably the filthiest thing I have in my arsenal that contains no actual curse words. I have NEVER had anyone actually be able to come up with a counter for this one. It stops EVERYONE in its tracks. Be warned though, the power of this insult comes in its shock value, if you use it on the same person multiple times, it will quickly lose its awesome effect, making it essentially a one-time use weapon. (delivery is important too, if you laugh during it, there's just no helping your sorry ass)
"If I wanted some sort of comeback from you... I'd scrape it from the roof of your mouth."
And for the people who think that using curse words as an insult is from lack of creativity, that's not always the case. Especially for people who go out of their way to make up and combine curses to make whole new and even more devastating words. I once heard a man string together, non-stop, right off the top of his head, every single iteration, combination, and bastardization of every curse word that I had ever heard, into a three minute long barrage of profanity. And it was beautiful.
And for all you colossal nerds who think that quoting Monty Python at someone is a massive blow to their ego, there's a special formula I want you to remember. For every time you quote Monty Python and the Holy Grail and someone else is around to hear it, your virginity is lengthened by another whole year.