complete the sentence as asked and state it directly then take it from there If you don't do that than you may show inability to folow directions.
Just to take a conrtrarian stance here - You need to answer the question but you dont have to say "My most signficant accomplishment to date is X". In fact, I'd argue that doing that just shows poor writing skills, a lack of imagination and a horribly boring and uninspiring start to what is likely already likely an essay that will have a lot of commonality across applicants. Yes, it says complete the following statement - but there are many ways to complete it without mindlessly regurgitating the first half back to the reader.
If someone says you "If you could only have one food in the wolrd what would food you have?"
You don't reply "If I could only have one food in the world, I would have pasta."
You just say "I suppose if I had to pick one, I'd pick Pasta"
Similarly "My most signficant accomplishment to date...." can be paraphrased in many different ways.
So, my 2 cents: Paraphrase. It shows understanding of the meaning of the question and offers at least a slightly less boring and mindless start.