Usually “dumbass” for me.
Edit: Apparently I can’t type 4 words without a typo.
We need to start us a club!
Maude.
Used when someone is poking along. Also used for anyone driving a Camry.
There was a Toyota Camry commercial from maybe twenty years ago, with four old grey-haired ladies in a Camry, at the beginning of a freeway ramp. One passenger says “Punch it Maude!”, and another says “Yeah, punch it!”
I’m sure the intent was that a Camry has enough power to accelerate onto a freeway, but to me it just said old ladies drive Camrys.
Punch it Maude.
I’m a fan of “sweetheart” as well. Works as insult and condescension for men, women, and the elderly alike.
Lately I’ve been using “numbnuts” a lot, for some reason.
You stupid asshole.
Nice use of directionals, asshole.
It doesn’t get any greener, asshole.
But ya know? I almost never use that word, otherwise.
Motherfucker
Chief
Grandad
(In order of most used)
I’m a cyclist, and sometimes when I say things out loud someone can hear, so I tend to mutter under my breath. Got cut up by a taxi the other day and the driver looked like Michael Caine, so I said some swear words and called him Alfie.
When I’m mildly irritated or there are children in the car: jackwagon or titwhistle.
When I’m really angry or alone: Any swear word or combination thereof is not only possible, but likely. 
Jackass and/or asshole.
If I really want to with the other driver ill, “Fuck you sideways with a rusty pineapple.” I may have picked that up on the 'Dope, over in the Pit.