if canon then full frame: 17-40mm f4 L
if crop canon then: 17-85mm IS
I like taking shots of the places I've been rather than the people in those places. So I choose a lens with decent wide angle and a little telephoto and also lightweight if I am going to carry it around all day.
I guess depende na sa imong type of photography. If you are still finding out what type of photographer you are or if you think you love to shot just about any subject then a lens with a wide coverage would suit you best and if you are on canon nothing beats the 18-200mm lens.