if you plan to lug them around all day (like i do ) you may develop strange criteria to decide on "favourite" focal lengths... take the 28/2--i love the lens, i hate the focal length. i'd much rather use 24 and 35, but the 28 is half the weight. luckily, this particular 28 has the most pleasing rendering of all wide nikkors... or the 105/1.8--another optical gem in an annoyingly useless focal length. what if my favourite lengths, 85 and 180, weigh three times that much? i end up packing the 105 and getting great frames from it... hey, maybe that's it--favourite focal length is what you shoot you favourite pictures with! if so, 28-50-105 for this humble oinker

ps. the "when" part is much easier--when it looks like it might look good a bit wider, i use the wide