Most alliums last well when cut and placed in water, smaller ones like chives and garlic chives can come out of the garden. Nearly any of the smaller ornamental varieties will also do very well. The bigger ones hold ok in a vase too, but are big enough to require special handling or they will just overpower anything else they are displayed with and can be real unwieldy in the process. Many of the flowers are also pleasantly fragrant, although the stems and leaves will smell oniony or garlicky. You pretty much have your choice of any of those that do flower, for cutting.