When looking at historical population trends, I personally prefer counties. They overstate populations slightly, but they're constant area once you get past the first few censuses.
1810
Allegheny, PA: 25,317
Hamilton, OH: 15,258
1820
Allegheny, PA: 34,921
Hamilton, OH: 31,764
St. Louis, MO: 10,049 (County includes the city until 1877)
Cuyahoga, OH: 6,328
Wayne, MI: 3,574
1830
Hamilton, OH: 52,317
Allegheny, PA: 50,552
St. Louis, MO: 14,125
Cuyahoga, OH: 10,373
Wayne, MI: 6,781
1840
Allegheny, PA: 81,235
Hamilton, OH: 80,145
St. Louis, MO: 35,979
Cuyahoga, OH: 26,506
Wayne, MI: 24,173
Cook, IL: 10,201 (created in 1831)
Milwaukee, WI: 5,605 (1835)
1850
Hamilton, OH: 156,844
Allegheny, PA: 138,290
St. Louis, MO: 104,978
Cuyahoga, OH: 48,099
Cook, IL: 43,385
Wayne, MI: 42,756
Milwaukee, WI: 31,077
1860
Hamilton, OH: 216,410
St. Louis, MO: 190,524
Allegheny, PA: 178,831
Cook, IL: 144,954
Cuyahoga, OH: 78,033
Wayne, MI: 75,547
Milwaukee, WI: 62,518
1870
St. Louis, MO: 351,189
Cook, IL: 349,966
Allegheny, PA: 262,204
Hamilton, OH: 260,370
Cuyahoga, OH: 132,010
Wayne, MI: 119,068
Milwaukee, WI: 89,930
1880
Cook, IL: 607,524
St. Louis, MO: 382,406 (this is city + county from here on)
Allegheny, PA: 355,869
Hamilton, OH: 313,374
Cuyahoga, OH: 196,943
Wayne, MI: 168,444
Milwaukee, WI: 138,537
1890
Cook, IL: 1,191,922
Allegheny, PA: 551,959
St. Louis, MO: 488,077
Hamilton, OH: 313,374
Cuyahoga, OH: 309,970
Wayne, MI: 257,114
Milwaukee, WI: 236,101
1900
Cook, IL: 1,838,735
Allegheny, PA: 775,058
St. Louis, MO: 625,278
Cuyahoga, OH: 439,120
Hamilton, OH: 409,479
Wayne, MI: 348,793
Milwaukee, WI: 330,017
1910
Cook, IL: 2,405,233
Allegheny, PA: 1,018,463
St. Louis, MO: 769,446
Cuyahoga, OH: 637,425
Wayne, MI: 531,591
Hamilton, OH: 460,732
Milwaukee, WI: 433,187
1920
Cook, IL: 3,053,017
Allegheny, PA: 1,185,808
Wayne, MI: 1,177,645
Cuyahoga, OH: 943,495
St. Louis, MO: 873,634
Milwaukee, WI: 539,449
Hamilton, OH: 493,678
1930
Cook, IL: 3,982,123
Wayne, MI: 1,888,946
Allegheny, PA: 1,374,410
Cuyahoga, OH: 1,201,455
St. Louis, MO: 1,033,553
Milwaukee, WI: 725,263
Hamilton, OH: 589,356
1940
Cook, IL: 4,063,342
Wayne, MI: 2,015,623
Allegheny, PA: 1,411,539
Cuyahoga, OH: 1,217,250
St. Louis, MO: 1,090,278
Milwaukee, WI: 766,885
Hamilton, OH: 621,987
1950
Cook, IL: 4,508,792
Wayne, MI: 2,435,235
Allegheny, PA: 1,515,237
Cuyahoga, OH: 1,389,532
St. Louis, MO: 1,263,145
Milwaukee, WI: 871,047
Hamilton, OH: 723,952