How Lincoln did in the slave states in 1860

In 1860, Abraham Lincoln was on the ballot in all the slave states north of the Missouri Compromise Line (Delaware, Maryland, Virginia, Kentucky, and Missouri). These were his vote shares in those states:

Delaware: 23.72%
Missouri: 10.28%
Maryland: 2.48%
Virginia: 1.13%
Kentucky: 0.93%

(In 1856, John Frémont had been on the ballot in the two cis-Potomac slave states, and got 2.12% in Delaware and 0.33% in Maryland.)

Lincoln carried only two counties in the slave states: Gasconade and St Louis Counties, Missouri ('both dominated by Germans').

We have some information about the nominees' vote shares at the county level for Virginia, Maryland, and Delaware.

Delaware

According to Wikipedia, these were Lincoln's vote shares in Delaware's three counties:

New Castle (28.12%)
Kent (26.75%)
Sussex (14.27%)

Breckinridge won every county by double digits. I am somewhat surprised at Lincoln's strength in Kent and Sussex Counties. 

Maryland

Megan McNish has created a map that distinguishes Maryland's counties by the vote share they gave Lincoln in 1860. For Lincoln in 1860, she uses ranges of 1% (hence, e.g., there is one colour for counties that gave Lincoln 1.1%-2.0%, one for counties that gave him 2.1%-3.0%, and so forth). Here are Maryland's counties by the rough vote share they gave Lincoln in 1860:

Allegany (12.1%-13.0%)
Garrett (12.1%-13.0%)

Cecil (4.1%-5.0%)

Harford (2.1%-3.0%)
Kent (2.1%-3.0%)
Montgomery (2.1%-3.0%)

Carroll (1.1%-2.0%)
Dorchester (1.1%-2.0%)
Frederick (1.1%-2.0%)
Washington (1.1%-2.0%)

All other counties gave Lincoln between 0% and 1.0%.

Allegany and Garrett--the two westernmost counties in Maryland--are outliers, giving Lincoln over twice as large a vote share as any other county in the state. No other county gave Lincoln over 5%, and aside from Cecil, on the border with Pennsylvania, no other county gave Lincoln over 3%. I found it somewhat odd that Dorchester, Kent, and Montgomery were relative areas of Lincoln strength, but at that level, the 'Lincoln strength' was low enough that it is probably not very meaningful that Lincoln landed one or even two 1%-wide ranges higher than in most other counties south of the Patapsco and Sassafras Rivers.

Virginia (and West Virginia)

In 1860, Virginia still included what would become West Virginia. The bulk of Lincoln's vote from Virginia came from the future West Virginia. County information for Virginia is from here (and in the calculations I make, I will be using the information from this document rather than from Wikipedia, to keep everything consistent).

Lincoln's best counties in Virginia (where he got over 3% of the vote) were:

Hancock (40.06%)
Ohio (21.39%)
Brooke (19.84%)
Marshall (9.56%)
Preston (5.94%)
Prince William (5.28%)
Wood (4.50%)
Mason (3.90%)
Monongalia (3.74%)

Lincoln lost Hancock County to Breckinridge by 1.26% or eight votes. 

Hancock, Ohio, Brooke, and Marshall are the four counties of West Virginia's northern panhandle. Except Prince William, all the above counties were in the future West Virginia, and all bordered at least one of Ohio and Pennsylvania. (Preston County additionally lies immediately west of Garrett County, Maryland.)

If I calculated correctly, Lincoln's vote share in the future West Virginia was 3.57% (1,805/50,555). His vote share in the rest of Virginia was 0.106% (124/116,746). Prince William County, where he got 55 votes, accounted for over 2/5 of his total vote in the rest of Virginia. The only other counties in the rest of Virginia where he got so much as half a percent of the vote were 'Alexandria City and County' (where he got 0.88%/14 votes) and Shenandoah County (where he got 0.52%/13 votes). 

Hancock County appears to have come the closest of any slave state county outside Missouri to voting for Lincoln in 1860, and in fact, it appears almost certain to be the only slave state county outside Missouri to come within even 10% of voting for Lincoln. Like St Louis and Gasconade Counties, Hancock showed a voting pattern evincing unionist sentiment. St Louis County voted Republican in every election from at least 1888 through 1928 (including for Taft in 1912), and Gasconade County has voted Republican in every election from at least 1888 through 2020 (including for Taft in 1912). Hancock County voted Republican in every election from at least 1912 through 1928 (including for Taft in 1912), turning Democratic only with the New Deal (and voting Democratic in every election from 1932 through 1996 apart from 1972). 

Edit: I later found this site which claims that Lincoln's vote share in 'Future West Virginia' was 2.8%, and in 'Virginia Proper' was 0.1%. There are also maps and figures of how 'Future West Virginia' and 'Virginia Proper' voted on other measures, such as secession and the 1830 constitution. The 2.8% is considerably off from my 3.57% (or vice versa), but it is still higher than the 2.48% Lincoln got in Maryland. (In 'Virginia Proper', it gives Lincoln's vote share as 0.1%, as I found.)

It also notes that Trump would have won 'Reunified Virginia' in 2016. It is interesting to note that, from 1872 (when Virginia first regained the right to vote after the War of Sections), Virginia and West Virginia have disagreed in 19 elections: 1896, 1900, 1904, 1908, 1916, 1920, 1924, 1952, 1960, 1968, 1976, 1980, 1988, 1992, 1996, 2008, 2012, 2016, and 2020. 1888 remains the last time that both voted for the same losing nominee; and 1948, 2000, and 2004 are the only elections since then in which both even voted for the same narrowly-winning nominee.