I haven’t traveled as much, but strictly as a tourist.
Going by tourist attractions and ease to reach them all

New York. World famous. Tons of places recognized from the movies and TV. Pizza to go to Heaven for.

Paris. Hundreds of years of culture packed into one city. Museums everywhere. Could spend years and not see it all.

Singapore. Very walkable place. Great subway system.

Package tour of Europe. Passed by London, Amsterdam, Geneva. Too short to get a feeling.

For Americans…

Washington, DC. Seat of US Power. All important activities and decisions through the history of the US made there.

Boston. Very compact, walkable city. Beginnings of the American Revolution.