Having spent a good deal of time shooting in Europe the past two summers, I would say that a 21 or 25 should absolutely be in your bag. 35 might suffice, but having the option of going wider is a big plus. Last year I used a 21, 40, and 90, and got good use out of all of them. Wides are necessary there, but I will advise that it can become a crutch. You don't always need to fit everything in the frame. Some of my favorite shots were taken with the 90, in fact. Then again, in some of those tight streets, even a 50 can feel like a 90!