"most significant reason" is correct and usable in written English.
It is typically used to describe the primary or most important factor that contributes to a particular outcome or decision. Example: The most significant reason for the company's success was its innovative marketing strategy.
It is found that particle collision is the most significant reason for particle rotation.
The most significant reason for E. Germany's situation is its subservience to the Soviet Union.
Vonn called that decision the most significant reason for her success in races like Friday's.
That Pettitte preferred to pitch here Monday night on his normal turn appeared to be the most significant reason.
By far, the most significant reason companies blocked social-networking sites was to prevent workers from slacking off.
But the most significant reason not to fear a repeat of the events of 2010 is that there is no longer an aviation "zero tolerance" policy to ash.
The DIT is filled out by the subject as follows: 1) firstly, the subject chooses the most suitable action (decision); 2) upon doing so, he or she then lists reasons for that action by degree of significance; and 3) lastly, the subject lists the four most significant reasons in order.
The article explains the most significant reasons.
"The 1999 World Cup was probably one of the most significant reasons we are here today, and we will all be proud of the players from the W.U.S.A. that will be representing their respective countries this September".
Officials said that North Korea's advancements in missile technology were among the most significant reasons that Mr. Panetta, as he approached the end of his tenure, had spent so much time in Asia.
One of the most significant reasons lies close to home: the absence of her sister – and the world's best female player – Serena, who is expecting her first child.
