In Spain I have no doubt with Rafael Nadal (tennis). The top6 will be completed with:
Pau Gasol (basketball): Silver at London 2012, bronze at Rio 2016. He defeated France at 2015 Eurobasket in one the best moment of Spanish sport of the decade + NBA career.
Mireia Belmonte (swimming): The best Spanish swimmer ever with 4 Olympic medals (1xGold, 2xSilver, 1xBronze).
Carolina Marín (badminton): Olympic champion and first woman with 3 World titles in a row.
Javier Gómez Noya (triathlon): Silver at London 2012 and 5 times World Champion.
Saúl Craviotto (canoeing): In Tokyo he could become the Spaniard with more Olympic medals.
In non-olympic sports, the best would be Marc Márquez (Moto GP).
The best team would be the men's national basketball team, followed by women's waterpolo and women's basketball.