The Callan Associates periodic table of returns shows how various asset classes have fared over the past 20 years. There is no "best" asset class. Even the lowly F Fund (LB Agg) dominated for 2 years (1990 and 2002).
http://www.callan.com/resource/periodic_table/pertbl.pdf
http://www.callan.com/resource/periodic_table/pertbl.pdf