Argentine Tango in
Philadelphia.

Argentine tango is the social, improvised tradition danced at milongas — distinct from ballroom tango in posture, embrace, and intent. It's a conversation in a close embrace, built on the walk and on listening, with its own music (the tanda/cortina structure) and its own floor etiquette. It draws a dedicated, patient community that runs separately from the salsa/bachata world.

Expect the longest learning curve on this list and don't let that stop you — every milonga was once everyone's first. Take a beginner course rather than drop-ins, watch before you dance, and learn the codes (the cabeceo, the line of dance); the community will help if you ask.