I’ve been asked this question a lot specially from friends who recently got into photography and started taking pictures. This is my preference and does not mean that my favorite is by any means the best option for you and your type of photography.
If I had to pick only one lens from the Canon lineup I would take the Canon 24-70 f/2.8L, it’s one of the sharpest zoom lenses that I have used and the range is good for many purposes. I personally would sacrifice some sharpness (read prime lenses) for versatility, just because I don’t want to carry around a bag of prime lenses to cover the range that one zoom lens can cover. I use this lens as my walk around lens and also my first pick if doing anything in my home studio, the bottom end (24mm) is good for wide angle shots and from 50 to 70mm it is amazing for portrait since there’s little to no distortion. Since I use a crop sensor camera (Canon 50D) I find myself using my Tokina 11-16mm for really dramatic wide angle shots but if you are a photoshop user, you can take multiple shots and stitch them together. I’ve even used the 24-70 for indoor sports where I didn’t need a longer zoom since I could get close to the subject, this is not very usual though and I would not recommend it for indoor sports with large fields such as soccer, basketball, hockey, etc.
Saying all that does not mean that the lens does not have any cons, the biggest hurdle for using this lens is it’s weight and that is expected for a fast lens with good zoom capability. It’s fine if you carry it in a backpack but if you want to hold it in your hand and walk around, you’ll find it heavy and painful after a while. And the other drawback for a lot of people just getting into photography is it’s price tag, which is reasonable for a professional L series lens but it’s not a mainstream/customer-oriented option.
So for a lot of people that want the best bang for their buck I recommend the Canon 50mm f/1.8 II. It’s an inexpensive fast lens with really good picture quality. Yes, you will lose the versatility of a zoom lens by opting for this one but you can’t beat it’s sharpness and bokeh with any other lens that are at least four times its price. But yet again, the best lens for any photographer is different depending on what they shoot.