n. — historical, Geography
an early 20th-century name for the Canadian prairies (see Image 1).
Type: 4. Culturally Significant — The last best West has its origins in the transfer of Rupert's Land and the North-Western Territory from the Hudson's Bay Company to Canada in 1870, and the ensuing Dominion Lands Act of 1872. The Act aimed to encourage settlement in the recently acquired Northwest Territories (out of which Alberta and Saskatchewan were created in 1905) and the newly created province of Manitoba, by providing free homesteads to eligible men from Britain, the US, Europe and Eastern Canada (see Canadian Encyclopedia reference for more information). Although the term was not used in the Act itself, the marketing campaign to promote the Act successfully used the phrase to entice potential settlers (see, e.g. the 1908 quotation). The campaign was hugely successful: between 1896 and 1914, two million people emigrated to Canada, transforming the Canadian West (see Library and Archives Canada reference "The Canadian West - Immigration"). The Canadian government also introduced block settlements, allowing entire communities to move to Canada together (see Library and Archives Canada reference "The Canadian West - Federal Administration").
The policy paralleled the US Homestead Acts, and the term last best West was inspired by the fact that the more easily accessible US West had already been largely settled.
The term continues to be most prevalent in Canada (see Chart 1), and retains cultural significance in the Prairie provinces (see, e.g. the 1994 and 2007 quotations).
See also COD-2, s.v. "last best West", which is marked "Cdn".
See also: Northwestern Territory Rupert's Land
- 1907  The Last Best West is CANADA WEST
HOMES FOR MILLIONS
GRAIN RAISING, RANCHING
MIXED FARMING, DAIRYING
FREE 160 ACRES FARM LANDS
WESTERN CANADA 
- 1908  The Last Best West Health, Liberty and Prosperity Awaits the Settler in the Prairie Provinces of Alberta, Saskatchewan and Manitoba - From eastern Canada, the United States, the British Isles and continental Europe farmers in thousands are yearly flocking to secure A Free Homestead of 160 Acres which the Canadian Government offers to every man over 18 years of age able and willing to comply with the homestead regulations. 
- 1922  Dr. G. E. Lloyd, the Bishop-elect of Saskatchewan, lectured the other day in London, England, on Western Canada, and in the course of his remarks said the last great Anglo-Saxon nation which the world would see is now developing rapidly on the prairies of Western Canada, an area nearly as large as all Europe, excluding Russia, and capable of supporting easily fifty million people. Dr. Lloyd called it "the last, best West," [....]
- 1955  Still the man said one thing which is correct: "The last best West." I think this is the last best peaceable place on earth to live today and I am staying. 
- 1978  The Canadian west, the last best west of the polemicist's dream, has long been placed firmly within the golden hue. 
- 1994  Speaking of which, you will notice a lot of Great Western items -- clothing, merchandize (sic!), meals etc. We pride ourselves on our Great Western heritage, here in the Last Best West. 
- 2007  His theory is that Saskatchewan and unbridled hope were intertwined from the start. It was sold to settlers as "The Last Best West," a place of such boundless potential that, the author says, "the myth is as strong today as it has always been." 
- 2013  With the election of Canadian Prime Minister Wilfred Laurier, a settlement campaign began - called "the last Best West," named because, by then, most of the best land in the U.S. had already been settled. This campaign led to, first a trickle, and then a flood, of settlers moving.
From 1896 until 1905, Canadian immigration was under the responsibility of Clifford Sifton, who was the minister of the Interior. Sifton instituted a massive program to encourage settlement through the establishment of immigration offices throughout Europe and the United States. Many different forms of advertising were used, including newspaper ads, brochures and posters that portrayed Canada as a prosperous nation with rich farmland in need of farmers. The benefits of moving to Canada were touted at agricultural fairs and at schools, and journalists were given free tours to write about.
Sifton also offered attractive incentives, including in some cases, free passage to Canada, free supplies to new immigrants, and most importantly, up to 160 acres of free farmland. 
Images:
Image 1: A promotional poster for homesteads in the Canadian West, 1907. Source: Wikimedia Commons. Photo: The Globe -- Toronto : Globe Printing Co.
Chart 1: Internet Domain Search, 31 Jul. 2013