I honestly think the reason why Aizen made stark Espada No. 1 was because deep down in their hearts they were both lonely, Aizen could empathise with Stark the most thus granting him position one in his army.
I don't think it had anything to do with power, loyalty or trust otherwise he would have made Barragan (arguably the most powerful) or Ulquiorra (most trusted and loyal to Aizen) number one.
I don't think it had anything to do with power, loyalty or trust otherwise he would have made Barragan (arguably the most powerful) or Ulquiorra (most trusted and loyal to Aizen) number one.