This is a hotly debated issue but I believe I know the answer.
In the USA, the best city flag is 100% the flag of Chicago.
However once you leave the states, there are some amazing city flags out there to compete with, mostly in Europe. Of all the city flags, I think Amsterdam has the best symbolism, color scheme, and overall feel. The city flag of Amsterdam captures the vibe of the city as well as does all of the other jobs of a flag by being versatile, simple, easy to identify at a distance, and well utilized by it's residents.
THE FLAG OF AMSTERDAM | DESIGN
Amsterdam's flag is derived from the city's Coat of Arms, just turned 90 degrees into a horizontal format. Thus, the flag has a red field with a black bar splicing it through the middle horizontally and within that center black stripe, three white St. Andrew's crosses (more commonly known as just white "X"s).
Quick christian history lesson for you, St. Andrew was one of Jesus’ followers, who was crucified on an X-shaped cross. Boom. Done.
WHAT DO THE THREE X MEAN?
First off, no, it's not denoting a strip club or the city's famous red-light district.
There are many legends around the meaning of the three Xs on the flag of Amsterdam. One says they are meant to ward off fire, floods and the black plague; however these symbols predate the black plague so it's obviously false.
Others say the black stripe represents the river Amstel and the Xs denote three specific places along the river. Other cities along the Amstel river use black to represent it in their shields this could be possible.
Most likely, the Xs (or St. Andrew's Crosses) come from the family crest of the Persjin family, who were prominent land owners in the area in the middle ages. Jan Persijn was even "lord" of Amsterdam for two years from 1280 to 1282. The Persijn family also owned two other villages near Amsterdam and their flags also feature St. Andrew’s Crosses, so signs point to this being the actual origin.