For maximum bang-for-buck and a kit that you can carry around (a lens you leave at home is not much use right?) it almost always makes sense to get three zooms - wide, normal and tele - and a couple of primes for special circumstances.
For landscape and architecture you will mainly use the wide and tele end. For wildlife only the tele, for portraiture mainly the 50-85 range and for street or party scenes mainly 17-40mm. Worth bearing in mind if your purpose would mean more lens changes.
Note I'm a nikon owner but I know what's available for the Canon system so I will give you two line-ups and you can take them for what they're worth.
Lighter zooms list:
A very short focal length but widely acclaimed as the sharpest wide angle this side of a grand. Speed is a nice bonus though not a major issue on a lens this wide.
On the nikon system this has everyone buzzing for its price/weight and good IQ. If you expect to move up to the 5D in the near future then get the
Canon EF 17-40mm f/4 USM L but otherwise this will do the trick.
A cheap light 5x zoom with good performance and IS. Slow but if you aren't doing sports or action you will not notice the difference for 99% of shots.
Heavier zooms list:Another great wide with a more useful, stay-on-camera zoom range.
Great range for a portrait lens in APC format cameras and DOF to match.
You shouldn't be able to get this IQ in a 5x zoom but you can, and it's cheap. Again, slow, but if you aren't doing sports or action you will not notice the difference for 99% of shots.
And finally with both kits throw in a
Canon EF 85mm f/1.8 USM for serious portraiture work with razor thin DOF.
http://www.photozone.de/canon-eos will help you a lot I think.