Aizen did know about Ulquiorra's 2nd release. In the manga, he specifically says that he had Ichigo fight Ulquiorra to develop his hollow powers, and the panel has Aizen envisioning Segunda Etapa Ulquiorra.
As for the number disappearing, Halibel, Barragan, and Starrk's numbers all disappear when they go Res, so saying Ulq is the strongest just because of that isn't right either. Also, Yammy sensed R2 Ulquiorra and when comparing their Reiatsu, he called it trash in comparison. Ichigo didn't disagree with that notion and sort of agreed, but said even so he'd still win. This is supported by Ichigo hitting Yammy with a Getsuga and him tanking it, when Striped Mask Ichigo drawing on his hollow powers should be the same power that he used against Ulq. The guides also state that Yammy really was the strongest. He even has a second rage boost/form ontop of the form that was already stronger than Ulq, and offscreen he was able to tag and damage Kenpachi and Byakuya, meaning he isn't as slow as people think. All that aside, Ulq said that there are 3 Espada that are stronger than him, and he never had knowledge of Yammy being Espada 0 as far as we know, so that would mean Halibel, Barragan, and Starrk, but if you wanna make the argument that he knew Yammy was Espada 0, then Barragan, Starrk, and Yammy would be stronger than him.
As far as the Espada rankings, in terms of raw power, they're pretty much in order. But if we're talking in terms of hax, then Barragan, Szayel, Zommari, and Arraniero are basically the only 3 with any kind of hax at all, while the rest are raw power and stat based.
From 9-4, there are clear differences in their levels base to base, but from 1-3, their strength levels were all so comparable that the shinigami present couldn't tell which of them were stronger based on their raw levels of reiatsu alone. Halibel, Barragan, and Starrk are all relative/comparable to each other, but Barragan has the better hax. That being said, when he decays things, they aren't instant, and energy/rieryoku based techniques, while they do age, can get through the respira before dissipating, as well as things like explosions, so if you wanna say Starrk, even with less hax, is stronger with firepower, you can make that case. Same with Yammy and Halibel potentially, though the latter specializes in water and there's no way to tell how fast water decays compared to normal raw reiryoku. In theory, someone like Zommari should be able to take out people comparable to him like Grimmjow and maybe Nnoitra with his ability should it land, but overall, the Espada rankings should be pretty accurate.
