Lucy,
for me there's no question: I use the Canon 17-55 F2.8 IS almost 90% of the time both in the studio and on location for pet portraits. The other 10% is divided equally between my 50mm F1.4 and the 85mm F1.8. The 17-55 is also my show formal lens.

For action photography, it's the 70-200mm F4L especially outdoors. Indoors, I go to the 85 for action stuff depending on how close I can get to the action.

Hope that helps.

Jim