The phrase "a landmark one" is correct and usable in written English.
It can be used to describe something significant or groundbreaking, often in the context of achievements, events, or changes.
Example: "The new policy is a landmark one in the fight against climate change."
Alternatives: "a groundbreaking one" or "a pivotal one".
The last year was not a landmark one for progress on sustainability here in the UK.
The commission responded: "To protect a landmark, one does not tear it down.
"To protect a landmark, one does not tear it down," the commission said in response.
This year's SDLP conference is a landmark one for the party founded out of the Northern Ireland Civil Rights movement at the start of the Troubles.
LIGHT SHIFT Not the best horse to pass through Warren Place, but her 2007 Oaks win truly was a landmark one, returning as it did her trainer to his rightful place at the top.
… The decision was a landmark one for a sport that, unlike the N.F.L., the N.H.L., the N.B.A. and Major League Baseball in the United States, has been reluctant to tinker with rules or customs on the field.
And more than four centuries after her birth, the Baroque artist Artemisia Gentileschi, who carved out a successful career against the odds, is given a landmark one-woman show.
The Australian government has announced a landmark "one-off" resettlement deal to the United States for some refugees held at Australia's remote offshore detention facilities on Nauru and Manus Island.
Each snapshot includes the vertical edges of the landmark (one or two, depending on the landmark) and some measure of the landmark's apparent width.
We hypothesized that in ultrasound method, the proper positioning of the guidewire would be less influenced by the direction of a guidewire J-tip compared to landmark one during SCV cannulation.
The building, although not designated as a landmark, is one of a handful remaining from the 1920's, and Mr. Longhitano said plans were in the works to get the building state and national landmark status.
