IMO, for a 1.6x crop body like the 50D, the best wide angle complement to the 24-105mm is the EF-S 10-22mm f/3.5-4.5. It's the only ultrawide zoom lens for a crop body, and it's an excellent lens (for example, it has much less barrel distortion at 10mm on a crop body than the 16-35mm f/2.8L II does at 16mm on FF, despite being equivalent in terms of angle of view). For the tele end, you can't go wrong with a 70-200mm lens, they are all great. Unless you shoot mainly action sports, if your budget permits consider the 70-200mm f/4L IS - it's a little better optically and has IS (but the f/4 non-IS version is a great value).