A 15 minute city is where everything is available to you within 15 minutes walking, biking, or public transit.
I know Mexico city fits this bill. Playa del carmen as well.
PDC I’d rather avoid. A bit too party tourism, dangerous I found.
Vallarta I’ve been too and almost fits. I enjoyed there.
I found Oaxaca to fit this perfectly while I was there for a couple months. I was staying in La Cascada but was able to walk anywhere I was interested in within 15-20 minutes. If you stay more central, even easier.