I'm not a big believer in the idea that a particular city dictates a particular lens choice. I think a lot depends on the specifics of your trip. I haven't visited Rome since 2012, and on that particular trip I shot mostly with a 24-85VR, and occasionally with an old Tamron 17-35 f/2.8-4 or a small prime like the 35mm f/2D. Most areas of Rome were very crowded on that trip, so I rarely changed lenses, except when I was out for early morning photo walks by myself. In the Vatican, it was so mobbed that changing lenses would have been almost impossible. But I came away with a set of photos that I was very pleased with, despite having used less than optimal lenses. My Flickr album from that trip is here:
Italy 2012, including Rome
In other more recent trips, I've done a lot of shooting in other cities with a mix of primes and zooms, or almost totally with primes, or dominantly with a single zoom (my general-purpose travel zoom is now the Sigma 24-105mm f/4). Each choice has pros and cons, but I feel I know how to adjust my shooting style to allow for the traits of the set of lenses I've brought. Here are some further thoughts:
1. If I'm making a trip where I think I'll have a number of opportunities to get out by myself to shoot for a couple of hours or more, I like to have my best primes with me, and I carry a set of f/1.4 primes at 24, 35 and 85mm (I shoot FX on a D800). I also usually throw in the 35 f/2 for those times when I want to go lightweight and attract less attention. I know I can do my best quality work with my primes, but most of my travel is not exclusively dedicated to photography, so I like to have a zoom, too.
2. These days, my Sigma 24-105 f/4 goes with me on almost every trip, and I like it especially for those times when I'm with a group and don't want to hold others up while I change lenses. But it's a very sharp lens in the center of the frame at all focal lengths, and gives me fairly satisfying results across its range of focal lengths and apertures. The main adjustment I make for it is that I don't expect great edge sharpness at 24mm, especially at f/4 or f/5.6, so I keep that in mind as I shoot. If I were shooting with it in the Colosseum, I'd probably try doing multi-image stitch panoramas at 35 or 50mm, rather than using it a lot at 24mm.
3. Sightseeing in a tour group usually suggests the need of a zoom, at least for me.
4. If I'm making a "mostly primes" trip, I usually select one or two lenses for the day, rather than taking my entire kit with me. Fast primes are heavy. Often, I'll use just one focal length on my outbound trip for the day, and then switch to the other lens on the return, especially if I know in advance that I'll be re-tracing some of the same route.
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Ray
My Flickr page:
http://www.flickr.com/photos/rritchie/