The various people they have asked have come up with the following suggestions:
Strada Dionisie Lupu
Strada Vadul Moldovei
Strada Arthur Verona
Strada Ion Brezoianu
Strada Ion Filitti
Some of these choices are bizarre: Brezoianu in particular is busy, dusty and a downright mess in places. Of those suggested, we would probably go for the obvious (Kisellef), not least for the museums and the park at the southern end. The startling number of derelict houses at the northern end would have to be ignored.
Having had a think about it, however, our vote for the most beautiful street in Bucharest would go to Strada Xenofon (which is here).
Strada Xenofon is the only street of its kind in Bucharest (at least we think it is, feel free to correct us): a staircase, basically, which leads from Strada Constantin Istrati up to Aleea Suter and the Carol Park Hotel. While hardly beautiful in the classic sense of the word, the street is charming and serves as something of a reminder that Bucharest (at least parts of the city) is actually rather hilly.