No question - the EF-S 17-55mm f/2.8 IS USM. I really enjoy my 100-400mm (for birds), love my 10-22mm (for landscapes and fun closeups), and my 70-200mm f/2.8L IS II it just an amazing lens!, but the most important shots, to me, are ones of my family - and the 17-55mm is the most useful for those, and it's an all-around excellent lens.