You should ask yourself, what are the subjects you like to photograph. From there you can derive the importance of a lens.
When I go out, my lightes kit is the 35mm f2 (on APS-C); If I'm out with the kids, I often take also the 70-200 f4IS with me, and when I know I will be indoors at some interesting place, I also take the EF-S 10-22 with me. Last year we did a 2 week cycling tour with camping; I wanted to go as light as possible, but the 35 would not have been felxible enough, so I brought my old 18-125 Sigma with me (no problem, the lightning was mostly good on the trip, and with good lightning most eqiupment makes good pictures).
So ask yourself what you like to do. My kit on a FF would be the 17-40 for indoors, cities and landscape, then the 70-200 for people and portraits, and finaly I would take a 50mm f1.4 with me. It always comes in handy, sometimes the light is bad, or if you go for a day trip where you like to be espacially light (and it fills the gap between the17-40mm and 70-200mm.