Question: why ulquiorra is the strongest espada?
Answer: Ulquiorra was easily the strongest Espada hands down. People forget the top 4 have all been stated to be Vasto Lordes, which means all 4 have very close power levels. Seeing as number 5 and below are adjuchas or gillian, that means 4 is closer in power to number 1 espada than number 5 is close to 4.