I have the usual big swiss army in the glove compartment and a Richartz knife/fork combo knife. But it's the knife for cutting the seat belt, breaking the windshield and self defence beside my right leg that you're interested in, I presume.
I change that one from time to time. Ideally it should be small enough for tactical purposes, large enough for smashing purposes, single edge[which rules out the Polkowskis] and relatively corrosion resistant[it gets wet in Canada] and a lanyard. Also, it can't be seen too easily by passenger or policeman looking in to write a ticket:it should be perfectly legal, but who needs a hassle?
My main knives there have been a nice little Freeman[no skullcrusher], CS Trailmaster[on the big side], Busse Steelheart II,[also a bit big] Wally Hayes custom tanto[this one a simple O1 brute] and, currently, a Lightfoot Predator[although it does have a sharpened top edge].