That depends on what you plan to do! Prime lenses are always sharper, but zoom lenses are more flexible. It you miss a shot because you are switching lenses than the zoom would have been a better choice. The weight is a second reason to limit what you carry in your bag. Zoom lenses are heavier so it might be better to carry a could of primes in certain situations.

For me sharpness of my primes are the main reason for using them but remember the contrast and saturation from a zoom lens can be post-processed to get similar look as the prime but you can’t add missing details. If you are only taking photos for the web you will be fine with a zoom for a resolution perspective.

Single lens only

If you are serious with your photography but only want or can afford one lens for some reason, you need to buy something more general like the Canon EF 24-105mm f/4 L IS, or the older work horse Canon EF 24-70 f/2.8 L, there are cheaper alternatives from Sigma that are ok, but than versions are really well built and will always work.

Canon EF 24-105mm f/4 L IS
A really good al round lens with includes image stabilization which come in handy in low light situations. This lens produces really nice images and is used by many traveling photographers and is very attractive as kit lens together with Canon 5D mark II. Total lens weight is 670g (without protection filter as in the image bellow), here on black background captured with Canon TS-E 45mm f/2.8 which is something very different than an al-round lens.

Canon EF 24-105 f/4 L IS, here on black background captured with Canon TS-E 45mm f/2.8 which is something very different than an al-round lens.

Canon EF 24-105 F/4 L IS is a very good al-round lens which is very attractive as kit lens available together with Canon 5D mark II.

Canon EF 24-70 f/2.8 L
This lens is the older version and can be a really good lens, easy to find on the the second hand market, since it is used by almost every photojournalist. Watch out for lenses that has been used professional for many years, they may need service, especial the iris may wear out. This lens is sharp enough for most assignment but a prime lens is even sharper. This lens performs better in the corners than the 24-105 listed above, less light fall off. Minimum focusing distance is small which is good but it is heavy, 950g

Canon EF 24-70 f/2.8 L II
A newer sharper version the lens above, expected to be an impressive high performing zoom lens, probably the best general purpose lens around for now. The new version is much more optimized and weights only 805 g. Not available yet, expected in July 2012. Expensive!

Canon EF-S 17-55 f/2.8 IS
A good budget lens, 645g

Canon EF 16-35mm f/2.8 L II
Very good universal wide angle zoom, very usable in low light situations du the the wide aperture and short focal length. This lens has a classical zoom range in you will find one in every photo journalist lens collection. 635g

Canon EF-S 17-85 f/4-5.6 IS
Good budget lens with a little longer zoom range. 475g

My recommendation here is to buy something that is good enough for what you plan to do, do not buy something over expensive that you do not need. It is easy to upgrade this type of flexible equipment due to the large second hand market. Also think twice about the lens weight, heavy equipment is a real problem while traveling. I you are lucky and have the financial options, always remember: A good high quality lens keep a lot of it value, crappy lenses do not.