Of the Vasto Lorde-class Hollows Gao Yu knew about, there were currently three in Las Noches.
The first was naturally Barragan, King of Hueco Mundo. As the future 2nd Espada, his strength was beyond question.
The second was Las Noches' scientist, the future 8th Espada, Szayelaporro Granz.
Szayelaporro had once been a powerful Vasto Lorde, even stronger than Barragan. But he had no interest in fighting; he sought only to create the perfect life-form.
So he served as a scientist in Las Noches.
After Aizen established the Espada, Szayelaporro was also the earliest 0th Espada.
However, Szayelaporro didn't care about combat strength—or rather, he even despised the fighting instinct in his own mind.
So he split off his older brother, Ilfort Granz, whom he had once devoured and who had been a battle maniac.
Since a portion of their soul had been split away, both individuals became much weaker. They fell from being the strongest 0th Espada to a level too low to even qualify for the Espada.
Later, Szayelaporro managed to rejoin the Espada and rise to the eighth position, while Ilfort served as one of Grimmjow's Fracción until his death.
At this point in time, Szayelaporro hadn't split himself apart yet. In other words, he still possessed the terrifying power of a Vasto Lorde.
The third Vasto Lorde was Pikaro, Number Three's "102." They were a group of children fused together. Aizen felt they weren't suited to being Espada, so he kicked them out and made them "Number Three."
But no matter how much they messed around, they were still Menos of the upper class.
So...
How should he put it? In the original story, Aizen actually had plenty of cards to play. He just didn't want to use them.
The Espada were certainly the ten strongest Arrancar