Choosing the best standard zoom lens upgrade is almost too close to call. The Canon EF-S 15-85mm offers an unparalleled zoom range with very impressive image quality and the latest 4-stop image stabilisation. The Canon EF-S 17-55mm has a more meagre zoom range but slightly superior image quality and the bonus of a fast, constant f/2.8 aperture throughout the zoom range.
Ultimately, it boils down to whether you'd rather have extra zoom range or the ability to get faster shutter speeds without increasing ISO in gloomy conditions, along with a tighter depth of field.
For us, the EF-S 17-55mm just about wins out. The Sigma 17-50mm f/2.8 is very close behind but features a more basic ultrasonic autofocus system that's slightly slower and lacks full-time manual override.
On a tight budget, Tamron's 17-50mm f/2.8 offers unbeatable value.
For a full-frame compatible lens, the Canon EF 24-105mm is the one to go for. It beats all the other full-frame lenses tested for image quality, and is the only one to feature image stabilisation.