Depends on the dog and the situation.
For regular stuff, Gavroche wears his Ella's Lead collar (he wears it all the time), martingale, leather euro leash, and seat belt harness. If we're doing something like hiking, all of the above plus a backpack. If we're going somewhere not in the car, no harness.
For everyday not-working things Logan wears a martingale, leather euro leash, and leather harness. If we're going somewhere where he's working, I'll sometimes put a vest or backpack under/over the harness. He also wears a Holt headcollar when he's working. Funny this topic comes up, actually, because just the other day I made a post on my blog detailing what he wears and the reason for each (if anyone's interested let me know and I can give you a link and permission to see his blog).