What about oxiracetam? Some of these sites list it as being more powerful and faster. Would that rank above phenibut?

It is certainly stronger and faster than aniracetam as a cognitive enhancer, but it isn't above (or even on the same scale) as phenibut. Where aniracetam is relaxing oxiractam is stimulating. People report enhanced motivation and concentration on top of the effects on memory and cognition. Because it is stimulating it can also produce agitation and insomnia if you use too much or too late at night.

Definitely worth considering if you are looking to play with nootropics in general.