Bit before the 20 year cut-off, but Romario and Marco van Basten are the top two finishers, for mine. Van Basten's career ended prematurely (he formally retired in 1995, but hadn't played for two seasons due to a severe ankle injury that reoccurred during his career and was made worse by a tackle from behind by Basile Bolli for Marseille against AC Milan in the 1993 Champions Cup final).
I remember watching Romario tear Manchester United apart in a Champions Cup game for Barcelona in 1994. Between 1985 and 2009, Romario scored 313 goals in 451 league matches, including 34 goals in 46 matches for Barcelona and 96 goals in 107 matches for PSV Eindhoven. He played for Brazil between 1987 and 2005 and scored 55 goals in 70 internationals.