Ours would be, not in any order:  #1radial tires on our tillage and planter tractors.  These came in 2002.  A huge improvement that we never saw coming, but we should have,  #2 Cruiser and Apron for our edible beans.  It has increase average yields from 25 bu/a to 35 bu/a.   #3 Seed technology and the understanding of what it takes to make a perfect field, agronomically and culturally, so you at least have a goal.  #4 grid soil sampling.  We had been running straight rates and the soil tests looked respectible, but better soil zones in our fields just didn't seem to produce better anymore.  What an eyeopener.  #5