I’ve tried google.com, but am not able to get a concrete answer.
In the United States? The world?
For the US, an incorporated suburb of any American city could qualify; there are many villages that were once just crossroads that now have populations of over 250,000. Rather than Las Vegas or Phoenix, I’l look at some of the megaburbs; Aurora, Colorado; Arlington, Texas; Mesa, Arizona; Henderson, Nevada and the like.
There are some good-sized but smaller incorporatedcities that emerged from what was literally nothing. Rio Rancho, New Mexico was a land scam in the 1960s, but now has over 50,000 residents. Same thing with Cape Coral, Florida; in 1950 it was a swamp; today 100,000 people live there. Technically the growth of those places was infinite.
I’ll WAG Phoenix, Arizona.
Are you asking about the city to get the highest population boost from 1900-2000, or are you asking about the city to experience the fastest growth spurt over a given period? Those will yield two different sets of educated guesses.
A wild-assed guess is useless unless the OP can define some parameters, namely:
-minimum size or existence of town in year 1900
-minimum size of city in year 2000
-growth in % terms or absolute growth?
I’d just off-the-cuff say Los Angeles.
Not a damn thing there practically 100 years ago. Now BOOM!
FWIW…
Las Vegas (Source)
1900 - 19
2000 - 475,000
(2,500,000% increase)
Phoenix
(Source
1900 - 5544
2000 - 1,321,045
(a measly
Darn it…Phoenix’s % increase is only about 23,700%. That is, if I did my math correctly.
Well no, there was a damn thing there practically a hundred years ago. Mission San Gabriel was established in 1771, and settlers moved there (LA) in 1781.
Not really. This is one of the things that always surprises Easterners. The big boom in Los Angeles was in the 1880s when the Santa Fe Railroad came in. By the 1900 census, LA had a cool 102,479. In 2000 it was 3,694,820. That gives it only about a 3600% increase.
Houston went from 44,633 to 1,953,631, for 4300%.
One big problem is that many of these cities have annexed large amounts of land area over the century, while eastern cities lost the ability to do that sometime early in the century. Looking at city boundaries today hardly mean anything. County or metropolitan statistical areas are far more indicative of what a city is today.
Los Angeles County has 9,519,338, the same as the Los Angeles-Long Beach Primary Metropolitan Statistical Area. But the Los Angeles-Riverside-Orange County Consolidated Metropolitan Statistical Area for 2000 is a full 16,373,645.
So which is really Los Angeles?
Los Angeles? “Real”? In the same sentence?!? 
I’m sorry, how did we get from Population Statistics to Baudrillard?
Sao Paulo increased from 240, 000 to around 11 million in 2000.
An 11 million person increase in 100 years would have to be close to the fastest growing city of last century.
And if you are after % increase: Canberra did not exist in 1900 (a few black fellas dont count because it was not a city) and in 2000 had a population of 311,000. So infinite increase.
Or how about Brasilia: Zero (1956) to 2 million (2000) in 44 years.
Hong Kong must have experienced a phenomenal growth rate between 1898 and 1999.
Tel Aviv was founded in 1909, and now has over half a million people; however, it was joined at some point with the ancient city of Jaffa, which already had a sizeable population at the turn of the century, complicating the math.
A quick internet search had suggested candidates not yet named here being Dhaka, Lagos, Kuala Lumpur, Mexico City and Riyadh.
I think in order to get a handle on the Q a little term-defining is still in order. Are we talking % growth or raw numbers gained? Also, are you looking for the largest cumulative gain measured either way starting in 1900 and ending in 2000, or the largest spurt measured per annum for any given year (or longer period) which occured during the 20th century, even if that city didn’t sustain it long enough to end up with the largest gain for the 100 year period?
A couple of quibbles:
(1) The white population of the area that became Canberrra would have been larger than the black population, since the blacks had already been pushed out by the white farmers.
(2) Canberra is not a city. Yes, really, since it’s not incorporated as a city. It’s local government is carried out by the Australian Capital Territory government, whichj looks after a lot more territory than just Canberra.
And I agree that the question is badly posed. Does “city” mean “incorporated local government area called a City”, or “metropolitan urban population area”? And does 0 to 1,000,000 count as a bigger increase than 1,000,000 to 20,000,000? I.e., is it percentage increase or absolute increase?
I’d wager Mexico City.
1950: ~ 3 Million
2000: ~ 18 million for the City itself, conurbation closer to 35 million.
Fair enough.
Doesn’t it have a cathedral - St Christophers?
Makes sense to talk about the city itself and absolute rather than relative increase. Both are clearer and easier to define.
This site (infoplease) rates the City of Mexico City as 8,705,100 in 2004.
Shanghai is the worlds most populous city with 13.3 million.