CREATE TABLE table_204_638 (id DECIMAL, "year" DECIMAL, "tournament" TEXT, "surface" TEXT, "round" TEXT, "winner" TEXT, "score" TEXT)

who won the most ?
SELECT "winner" FROM table_204_638 GROUP BY "winner" ORDER BY COUNT(*) DESC LIMIT 1
SELECT "winner" FROM "table_204_638" GROUP BY "winner" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1