Traveling alone changes what you want from a neighborhood. You need places where showing up alone at a bar is normal, where cafés don't treat solo diners like a problem, where you can walk safely at 11 PM, and where there is enough street life that you never feel like the only person out.
Our shortlist of neighborhoods that do all of the above, in rough order of how easy they are for first-time solo travelers:
Europe: Le Marais (Paris), Neubau (Vienna), Vesterbro (Copenhagen), Kreuzberg (Berlin), Trastevere (Rome), Bairro Alto (Lisbon), Södermalm (Stockholm).
Asia: Shimokitazawa (Tokyo), Seongsu-dong (Seoul), Tiong Bahru (Singapore), Nimmanhaemin (Chiang Mai).
Americas: Williamsburg (Brooklyn), Palermo Soho (Buenos Aires), Mile End (Montreal), Condesa (Mexico City).
What these places share: excellent café density (so you are never far from a reason to sit down), good public transit (so you can get home easily), a mix of locals and travelers (so you are not the only tourist), and an active street life that continues after sunset.