I know that Meta mostly covers SO's functionality and regulations, but say I had a question like "what's the most popular question with no answers on SO?" Where would that belong, Meta, or SO?
It would belong here on meta. All questions about, concerns with, support for, discussion of, and odes to Stack Overflow are off-topic on Stack Overflow, but can be posted here.
That said, normal rules apply: do your research, make sure you provide enough information (define "popular"), be polite and respond to any requests for clarification.