Douchebag is my favorite and all it's variations. Douchefck. Bilbo Douchebaggins. Also, fcktard is a classic- especially with stupid drivers, they are all fcktards.

I like reversing things too... Instead of saying "do work, son!" I'll call people "daughter". When someone tells you to "Shut Up!", you can respond "Open Down!". Finally, you can turn any 'your mom' diss around:

1) harmless comment "I hate this nail."
2) "Your mom's a nail"
***3) "I'll nail your mom"

It works every time. Even it it doesn't make sense when you reverse it from "Your mom's ___" to "I'll ___ your mom", it dumbfounds them into not having a response.

One's I've been favouring lately:

Nervous: My arsehole's twitching like a rabbit nose
Ugly-She's got a face like a slapped arse
Annoying-He makes my shit itch

All charming traditional English phrases

a simple: "Your a Prick!"
does it for me.
im good with my comebacks :D