Yeah, Les is cheating, working medium and large format into the answer in the 35mm forum :P
you know, this question doesn't mean what it used to, what with the incredible zooms available today. you could say a 16-35, a 24-70 and a 70-200, all f2.8, of course.
unless you get into the nuances of optical performance among or within brands, the answer is usually something inclusive (28mm), something fast (50mm), and something to access from a distance, that will also let you do portrature (90, 180?).
Another perennial (sp?) question is the one camera, one lens question. The answer is often even a camera you don't own, because it's a compromise compared to the combination of cameras/lenses you do own. this question is often driven by the realization that when every camera/lens you ever owned isn't enough, maybe the answer is to reduce the variables and get back to concentrating on the picture, instead of the equipment.
BTW, remind me not to let Les do my portrait with his 35mm lens on his medium format camera!