Mmmh … let me think. We definitely tended to play more upbeat, louder and more well known songs. “Punchy” tunes by the Beatles, Bob Dylan, Simon & Garfunkel and contemporary stuff always worked best. Well-known songs with lots of harmonies always caught people’s attention.
Especially when we were playing without amplification (in most cases) intricate finger-picking stuff or quiet ballads pretty much got lost in the noise of the street. Some examples: Mrs. Robinson (Simon & Garfunkel), Help (Beatles), I’m Yours (Jason Mraz), These Boots Are Made For Walking (Nancy Sinatra), …
Busking is a great way of learning what’s effective and what makes songs stand out. In our experience, it is less ideal for working on subtle techniques and complicated details, at least not on busy roads.
Those experiences have taught us a lot plus it was a fun way of earning some extra pocket money 😉