Canon EF-S 17-55mm f/2.8 IS USM
This is my always-on lens. In the photo-bag it is always armed with flower-hood, attached to the body, ready for shooting.
Image quality is superb, build quality is decent and it´s vesatility is amazing. Almost never miss it´s predecessor´s additional 30mm at the far end.
2.8 plus IS opens up new dimensions in available light photgraphy. Even wide open it´s sharp enough for almost any use.