which US city has good public transport other than NY

Basically the title.

I want to do a three week trip to a big US city but I hear that for most cities, it is best to have a car. I looked at renting a car but it is pretty expensive and rather spend my money on something else.

New York is a bit too expensive for me too go for three weeks.

If anybody has any experience with public transport in the US or travel around some big cities it would very much be appreciated.


Edit: This post blew up holy sh*t. Thanks everyone for the nice messages and responses. I still need to read through most of them but I am going to give NY another look.

I want to clarify. I want to stay in one city for three weeks because I want do something specific there. I want do a small training camp right before I do a big competition back home.