To paraphrase a response in one of the Flickr groups to this very question, in a hypothetical ideal world I'd like the:
Canon EF 10-1000 f/1.0 L DO IS USM GTi PDQ RTFM WTF
It's a beast of a lens that's suitable for every possible situation you can think of. It's lightweight because of the diffractive optics lens technology, is weather and vacuum sealed, and only weighs 227g! Remarkable stuff.
In the real world, however, I made do for a long time with a Sigma 28-300 f/3.5-63 Aspherical IF DL Hyperzoom (whatever all that means, and it's taken from the lens body, not made up this time!). The range is truly fantastic (especially on the crop sensor of the D30 and similar.
The downside is the variable, and not particularly large aperture, and somewhat slow autofocus. And the lack of IS is a pain, too.
On my 5D, however, there's some noticeable distortion at the 28mm end, and chromatic aberration if you peek close enough.
As a cheap-ish starter, all rounder, if-you-can-only-carry-one-lens jobbie, though, it's pretty cool.