The phrase "most significant decline" is correct and usable in written English.
You can use it to describe a decline that is more significant than other declines. For example, you could say "Sales saw the most significant decline in the last quarter compared to all other quarters."
The New Haven area has witnessed the most significant decline in competition.
Women with mildly reduced kidney function who consumed diets high in protein experienced the most significant decline in GFR.
Yet when he leaves Downing Street in May, he will have presided over the most significant decline in British influence in Europe for a generation.
CBS's entertainment division showed the most significant decline, producing $1.71 billion in revenue compared with $1.84 billion in the same period last year.
The California county that has seen the most significant decline in crime in the past decade has been San Diego - which has a very low incarceration rate compared to other counties.
During his time in Downing Street, David Cameron has presided over "the most significant decline in British influence in Europe for a generation", the shadow foreign secretary, Douglas Alexander, has claimed.
The most significant declines happened during the Obama administration, something Trump has failed to acknowledge.
It increased most dramatically among 18-to-24-year-olds, who had previously shown the most significant declines.
Among those with the most significant declines, five were financial institutions, including Berkshire Hathaway.
The most significant declines can be expected in the south and centre of the country.
Reporters Without Borders writes that the U.S. faced "one of the most significant declines" in the world last year.
