Also, if my memory does not betray me, the Espada are ranked based on the strength of their Reiryoku or Reiatsu. Starrk is the best explanation for this. Simply by being near other hollows, his Reiatsu destroyed them. That was simply due to the quality of his Reiatsu being enough to exterminate them. The reason people make the mistake that Espada rankings are based on strength (assuming I myself am not mistaken), is because of the way each Espada higher up the rankings was stronger. Typically, the stronger you are, the stronger and more powerful your Reiryoku is going to be, which will in turn increase the quality of your Reiatsu. Ulquiorra may have been stronger than the rest, but Starrk's Reiatsu had the potential to kill. Granted, those hollows were either generic hollow or possibly Adjuchas, but the fact that simply being close enough to Starrk was sufficient to kill them is still quite impressive. Another reason Espada rankings are more than likely based on Reiatsu quality/strength is one of the worst and my most hated of the Espada, Yammy Llargo. Yammy was barely strong enough to make it into the Espada, that is clear since his rank is 10. While his strength certainly does take drastic jumps in his Resurreccion state, due to the trait of this release state being associated with his growing anger, it still is not that impressive. In his early stages of Resurreccion, he was only cut by a Getsuga Tenshō Hollowfied Ichigo in Bankai state. At first glance, this may be quite the feat, given that since he was only ranked 10, he was able to resist damage from a Getsuga Tenshō that well. However, reading later chapters, and referring back to Ichigo's battle with Ulquiorra and Grimmjow as well, it is borderline laughable.
During Ichigo's travel back to Karakura Town with Unohana via Kurotsuchi Mayuri's Garganta, Unohana remarked upon his Spiritual Energy. Ichigo retorted by saying that it was only that weak due to how little was left of his Shihakushō in the Bankai state. At this point in time, Ichigo had barely a sleeve remaining of his Shihakushō. That means that his Spiritual Energy was barely at 1/3 of it's full strength, and that was when Ichigo was not Hollowfied. Upon making that statement, Unohana had originally thought that even with that little remaining of Ichigo's Bankai Shihakushō, his Spiritual Energy was already comparable to that of a Captain's. That is actually really scary, considering that it was barely 1/3 of his full strength while not Hollowfied. And for Yammy to be nicked by a Hollowfied Ichigo in Bankai state that was at 1/3 of his full power is quite lucky for him, since a full power Getsuga Tenshō from a full power Ichigo would have sliced his head clean off.
In Ichigo's fight with Ulquiorra, Ichigo was Hollowfied, and his Bankai Shihakushō was a little damaged, so he was pretty close to full strength. Upon firing a Getsuga Tenshō at an Ulquiorra that was in his first Resurreccion, it was completely ignored. Ulquiorra blocked it without so much as a scratch, and with the utmost nonchalance. Grimmjow demonstrated a similar feat, (although Ichigo was stronger during his fight with Ulquiorra, compared to his fight with Grimmjow) being able to block a Getsuga Tenshō from a Hollowfied Ichigo in Bankai state. Both combatants were at or near full strength, so I actually think this feat from Grimmjow is more impressive than the feat Ulquiorra demonstrated.
It seems I got carried away with that.
TL;DR Version.
Espada rankings are based upon Reiatsu quality/strength.