@mortonjt summed it up pretty well -- I don't know of an always-perfect way to select a log-ratio for these sorts of analyses, although there are plenty of solid solutions like selbal, amalgam, etc. that won't require you to mess around with Qurro, biplots, etc.
...This does bring about the worrying realization that there isn't necessarily a single "correct" way to analyze a dataset. In theory someone could go through hundreds of different differential abundance tools, different tool parameters, different ways of selecting log-ratios in Qurro, etc. until they find the "one" setup that gives them the results they'd want to see. This is clearly a bad idea, since if they go that far then the odds are likely that the results seen will be noise at that point (...plus it violates the scientific method pretty hard, I'd think). An antidote to this, I think, is being very explicit about what they have done and why they have done it, and being willing to accept that there may not be anything interesting going on in the dataset 
(...which probably doesn't happen often enough in microbiome research, but that's a paragraph for another day).
In general, looking at features pointing in different directions in the biplot is useful, like @mortonjt mentioned above. "Autoselection" in Qurro can be useful for this if the samples you are attempting to separate are separated along a specific axis in the biplot (for example, gut samples vs. other samples in this tutorial), but this isn't always the case.
In the biplot you posted, are you trying to find a log-ratio that distinguishes the red-colored samples from the blue-colored samples? You could try autoselection along Axis 1 to do this, since these samples seem to be kind of separated along Axis 1, although I would hesitate to assign "significance" to the results because it looks like there are only three red-colored samples. (Also in general I'm not a huge fan of using p-values for the results of exploratory analyses, but that's a whole other holy war...)
If you're looking for further reading, I wrote a pretty in-depth comment about some of the details on selecting features from DEICODE in Qurro a few months back: it's a bit long, but the section starting with "To be sure, I would like to ask you" might be a good place to start.
Hope this helps clarify things!