Florida, Illinois, Missouri, North Carolina, Ohio primary bellwether counties

[Nym90]

Note that some lists below are of only the counties that haven't voted for the winner in every election as opposed to all that have, since the list of those that haven't was sometimes much shorter.

Florida:

For the Dems, the following are the only counties that haven't voted for every statewide winner since 1992:

Alachua   
Baker   
Bradford   
Duval   
Escambia   
Gadsden   
Gulf   
Hamilton
Holmes
Jackson   
Jefferson   
Lafayette   
Leon   
Liberty
Madison
Martin
Palm Beach
Seminole
Suwannee
Taylor
Union
Washington

GOP counties that have backed the statewide winner since 1992:

Bay   
Brevard   
Broward   
Charlotte   
Citrus         
Hernando   
Hillsborough   
Leon   
Manatee   
Martin   
Miami-Dade   
Monroe   
Okaloosa   
Orange   
Osceola   
Palm Beach   
Pasco   
Pinellas   
Polk   
St. Lucie
Sarasota
Volusia

Illinois:

For the Dems, the following are the only counties that haven't voted for every winner since 1992:

Calhoun
Champaign
Dupage
Franklin
Gallatin
Hamilton
Hardin
Jefferson
Johnson
Lake
Lawrence
Massac
Perry
Pope
Saline
Union
White

For the GOP, counties that have voted for the winner in every election since 1992:

Boone
Champaign
Coles   
Cook   
DeKalb   
De Witt   
Douglas   
DuPage   
Grundy
Kane   
Kankakee   
Kendall   
Lake   
LaSalle   
Livingston   
McHenry   
McLean   
Macon   
Menard   
Morgan   
Peoria
Piatt
Putnam
St. Clair
Sangamon
Stark
Will
Winnebago

Missouri:

Dems bellwether counties since 1992:

Boone
Cole
Jackson
Nodaway
St. Louis city
St. Louis county

GOP bellwether counties since 2000 (no data for 1996):

Atchison   
Audrain   
Bates   
Caldwell   
Camden   
Carroll   
Cooper   
Daviess   
Franklin   
Gasconade   
Gentry   
Grundy   
Harrison   
Henry   
Jefferson   
Johnson   
Lafayette   
Livingston   
Montgomery
Nodaway   
Osage   
Perry   
Pettis
Pike
St. Francois
St. Louis county
Ste. Genevieve
Saline
Warren

North Carolina:

Dem bellwethers:

Alamance   
Anson   
Beaufort   
Bertie   
Cabarrus   
Caswell   
Chatham   
Craven   
Cumberland   
Durham   
Edgecombe   
Forsyth   
Franklin   
Gates   
Guilford   
Halifax   
Hertford   
Hoke   
Jones   
Lenoir   
Martin   
Mecklenburg   
Moore
Nash   
New Hanover
Northampton
Onslow
Pamlico
Pitt
Rowan
Sampson
Scotland
Vance
Wake
Washington
Wayne
Wilson

GOP bellwethers:

Every county has voted for the winner in every primary since 1992. North Carolina was always late enough that the race was de facto over by the time it voted.

Ohio:

For the Dems, the following are the only counties that haven't voted for every winner since 1992:

Cuyahoga
Delaware
Franklin
Hamilton
Montgomery
Trumbull

GOP counties voting for every winner since 1992:

Butler
Clermont
Cuyahoga
Delaware
Erie
Franklin
Geauga
Greene
Hamilton
Lake
Lorain
Mahoning
Medina
Montgomery
Portage
Stark
Summit
Trumbull   
Warren

[RI]

If you're looking for some specific counties to watch for margins, some good ones (with alternatives) are:

Dems
Florida: Pinellas County (Seminole)
Illinois: Carroll County (Morgan)
Missouri: Cole County (Platte)
North Carolina: Anson County (Lenoir)
Ohio: Butler County (Summit)

GOP
Florida: Pasco County (Orange)
Illinois: Peoria County (McHenry/Macon)
Missouri: St. Francois (Bates)
North Carolina: Ashe County (Camden)
Ohio: Mahoning County (Trumbull)