It's a bit of a broad question, but I'd recommend the 24-70mm F/2.8 L USM but that's not very wide on that cropped sensor body for indoor shots. The quality is there, and speed for indoor shots, but not very wide at 24mm on cropped sensor is 38mm (35mm equiv). It's also pretty huge and heavy and might seem silly on the small camera body. I'd suggest a battery grip if you're going to use this lens on that body. The big flash will help balance it out a bit though.
If it were my money though, that'd be what I'd go for.
You're looking for quality, and while other brands have great quality lenses, Canon's "L" lenses (white, with red ring for telephoto lenses, or black with red ring for shorter lenses) these are Canon's flagship lenses, they cost a lot but are considered to be generally the best. They're Canon's professional lenses range.
For shooting indoors you need a fast lens, they are very expensive, big and heavy, but look for anything with the smallest F number you can afford. F/2.8 is a fast lens, F/4 isn't particularly fast, especially indoors, f/1.8 is fast but at f/1.8 and below your depth of field is likely to be too limited for most semi-close-up indoor shots.
IS (image stabilisation) is great, but remember it only helps keep you still. It won't freeze your subject. Also, don't trade off lens speed in favour of IS. An f/2.8 lens without IS is preferable to an f/4 lens with IS. The IS just makes up for the fact that f/4 won't allow a sufficiently fast shutter speed indoors. As said, IS won't keep your subjects still.
USM is great too, it focuses much quicker than non USM lenses, which is a huge benefit in lower light levels where slower focusing mechanisms might never focus in time to get the shot.
Also, on the subject of focusing, this is where faster glass helps with AF too. Because a lens with a max aperture of f/2.8, is f/2.8 until you press the shutter button. So even if you stopped down to f/5.6 say, for depth of field, it's still f/2.8 while focusing, so the camera gets more light to focus with, so focusing will be faster. An added bonus is that your view-finder will be f/2.8 until the shutter button is pressed too, helping you see through the view-finder better.
When you've set your mind on a lens, take your body to your nearest camera shop and give it a try. Put the lens on the body and have a play with it.
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My photos:
http://www.flickr.com/photos/57315163@N02/ 