[yorwba]

I asked "Which country has the most subway stations?" and got the query

  SELECT ?country (COUNT(*) AS ?stationCount) WHERE {
    ?station wdt:P31 wd:Q928830.
    ?station wdt:P17 ?country.
  }
  GROUP BY ?country
  ORDER BY DESC(?stationCount)
  LIMIT 1

https://query.wikidata.org/#SELECT%20%3Fcountry%20%28COUNT%2...

which is not unreasonable as a quick first attempt, but doesn't account for the fact that many things on Wikidata aren't tagged directly with a country (P17) and instead you first need to walk up a chain of "located in the administrative territorial entity" (P131) to find it, i.e. I would write

  SELECT ?country (COUNT(DISTINCT ?station) AS ?stationCount) WHERE {
    ?station wdt:P31 wd:Q928830.
    ?station wdt:P131*/wdt:P17 ?country.
  }
  GROUP BY ?country
  ORDER BY DESC(?stationCount)
  LIMIT 1

https://query.wikidata.org/#SELECT%20%3Fcountry%20%28COUNT%2...

In this case it doesn't change the answer (it only finds 3 more subway stations in China), but sometimes it does.