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.
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.