Landscape and Cityscape? You can't go wrong with this: the EF-S 15-85mm http://www.amazon.co.uk/Canon-EF-S-1.../dp/B002NEFLDM

The 24-105mm won't be wide enough for a lot of landscape shots on the 450D, and the 17-40mm I don't think gives enough zoom range.

The Tamron 17-55mm f/2.8 non-IS is highly regarded too.