Which of the four border states in 1861 (Delaware, Kentucky, Maryland, Missouri) was the easiest for the Union to maintain and which was the hardest?

I like the above response and think a good case is made
for Missouri.  I would argue that Maryland was more difficult to maintain in the Union. 
While Missouri was distant, a frontier state essentially, and the American/Union
military presence there was minimal, there was not the population of Maryland in either
whites or slaves, nor the danger to the Union if Maryland had
seceded.

If Missouri had joined the Confederacy and the
Union had not stopped them, the worst that would have happened would have been a delay
in eventual Union victory.  They still would have advanced from Illinois and/or Kentucky
and occupied the Mississippi River, thus isolating Missouri regardless.  If Maryland had
seceded, the capital of the country would have been surrounded geographically by the
Confederacy, and under siege.  At the least, it would have had to be moved to New York
or Boston, and this may have prompted British aid to the
South.

Furthermore, Maryland had a very pro-secessionist
population that gave volunteers, aid, comfort and supplies to Lee's armies as they
marched North.  Keeping them in line was definitely harder.