What is the MOST important quality for a species or individual to survive?

Adaptability?

Species co-evolve in terms of each other all the time. Any quality may suddenly become essential because of the evolution in another species. Similarly, a quality which was essential can become irrelevant. An example might be the effects of the meteor or comet that caused the extinction of the dinosaurs. The qualities which had made them dominant for 150 million years became worthless. The same variability can apply to individuals. Some species can survive radiation quite nicely while most cannot. Some can survive drought better than others. None of these qualities are more important than the others until circumstances make them so. In short, your question could only be answered with a perfect knowledge of the future.

Not dying :-)

Someone answered adaptability, and that is a great answer ... for a species. It doesn't fit (biologically speaking) for individuals. That is, one would be equivocating on the meaning of the word "adapt".

Someone else answered breathing, and taht is a great answer ... for species that breathe. But lots of species don't.

In many cases, species that produce a lot of offspring have a better chance of surviving than those taht don't ... but that doesn't apply to survival of an individual.

And the basic characteristics of life don't make much sense as answers to the question either: metabolism is required for an individual to survive, but it doesn't make much sense to apply that to a species.

Being able to breath?