Unquiorra was by far the strongest Espada, even in his first release.
It honestly doesn't make sense though, I mean, we saw him stomp Bankai/Mask Ichigo before he even released his sword. He only released it because he wanted to show Ichigo the difference between them, and then only released the second form because he Ichigo was still holding out hope of winning and Ulquiorra wanted to crush that hope completely.
None of the other espada showed anywhere near that kind of power. Ulquiorra was quite frankly ridiculously powered, and Aizen knew this, that's why he entrusted everything to Ulquiorra. Ulquiorra was basically his confidant and go to guy, and the power he displayed was well beyond any of the other espada. Heck, the guy's spiritual pressure was described as being like an ocean of water as opposed to being just mere energy, it was so thick and heavy. Ridiculous.
Ps. I don't think there is such a thing as segunda etapa. It's obvious that Ulquiorra was doing something similar to what Yumichaka did with his release. The first release is probably a half release, and the second one is the full release.
Ulquiorra probably didn't realise that his initial release was a failed release and then when he mastered his ressurrection eventually, he concluded that it was a second release whereas it was actually his proper, full release.
Skilled and smart as he is, the guy clearly hadn't mastered his powers, because even in segunda etapa, that exploding javelin he throws, he admitted that he hadn't mastered it properly and therefore couldn't aim/direct it properly and could end up harming himself.
His power was too much so he hadn't yet learned to control it I guess.