Of course the most obvious is "West Virginina". Almost 1/3 of the Southerners were against it and 10% (almost 100,000 whites and 100,000 blacks) of the Southerners fought on the northern side. Tennessee for example never passed a free vote to join the confederacy;
http://www.acu.edu/sponsored/restoration_quarterly/documents/RQ_40.2_(Holley).pdf
Restoration Unionists in the South are overlooked because Southern
Unionists in general are overlooked. Although as many as one-fourth to
one-third of the people of the South opposed secession and about ten percent
of the available manpower of the South fought on the Union side, 1 they are
scarcely remembered.
Tennessee had the largest loyalist population. The first vote on secession
failed, and the second, positive vote was obtained by the use of force. Much
of east Tennessee and four counties in midwest Tennessee continued to vote
to remain in the Union even on the second vote.
Another group of Unionists almost totally forgotten are the Southern
Blacks. They furnished about 100,000 soldiers to the Union Army, or about
the same number as Southern Whites.