I'm surprised nobody has suggested a zoom. I find a 28-70mm pretty handy for most scenes you stumble upon. Often you really don't have time to change lenses and I hate carrying much for this kind of shooting anyway.
I think zooms are most useful when you are stuck in one spot; when you are free to roam, their biggest advantage is lost. And you pay with size and weight (and expense, if they are a very good quality zoom, which one needs to come close to the quality of even the fixed-length lenses that are not necessarily high-end glass). Wide zooms are often an exception, as they are not too huge or too heavy. However, they are still larger than fixed-length wide, and good ones are expensive. They are also not available in very fast versions (f/2.8 is as fast as they get TMK), and I think fixed-length wides are generally optically superior. Having something small, light, fast, cheap, and simple trumps the ability to change focal lengths on the fly IMHO.
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