This is a pet peeve of mine. When someone says "what's the
most important thing?", the answer can't be "every thing is most important". We're being asked to prioritize by someone who's looking for an entry point. Even if it's wrong (or at least not the most correct) answer, we should give him a concrete answer.
To answer the OPs question, I'd say that data structures are the thing that I'd consider most important. Especially nested structures (Arrays-of-Arrays, Hashes-of-Arrays, Arrays-of-Hashes, Hashes-of-Hashes, etc). Master this, and a lot of other things will become easy.
thor
Feel the white light, the light within
Be your own disciple, fan the sparks of will
For all of us waiting, your kingdom will come