I own lots of lenses, but only one zoom, which I like to use for photographing people at events and such, and it's a razor sharp, constant aperture lens that will work on all your camera bodies--Tamron Adaptall II SP 35-105/2.8 ASL.
Downsides--a little barrel/pincushion distortion at the extremes of the range, but this is true to some extent of all tele-wide zooms (the inherent design tradeoff in a zoom lens of this range is usually either distortion or field curvature), and I wish the push-pull zoom were a little smoother, but you can get used to it. There is a later version with a slightly wider zoom range, but I'd stick to the 35-105, because I think one invites more problems by widening the zoom range.