Here are the top ten of the US Civil War. I think that the artillery had a lot to do with high casualty rates. # 5 was the bloodiest single day battle of Antietam.
http://americanhistory.about.com/gi/dynamic/offsite.htm?site=http://www.civilwarhome.com/Battles.htm
#1
Battle of Gettysburg
Date: July 1-3, 1863
Location: Pennsylvania
Confederate Commander: Robert E. Lee
Union Commander: George G. Meade
Confederate Forces Engaged: 75,000
Union Forces Engaged: 82,289
Winner: Union
Casualties: 51,112 (23,049 Union and 28,063 Confederate)
--------------------------------------------------------------------------------
#2
Battle of Chickamauga
Date: September 19-20, 1863
Location: Georgia
Confederate Commander: Braxton Bragg
Union Commander: William Rosecrans
Confederate Forces Engaged: 66,326
Union Forces Engaged: 58,222
Winner: Confederacy
Casualties: 34,624 (16,170 Union and 18,454 Confederate)
--------------------------------------------------------------------------------
#3
Battle of Chancellorsville
Date: May 1-4, 1863
Location: Virginia
Confederate Commander: Robert E. Lee
Union Commander: Joseph Hooker
Confederate Forces Engaged: 60,892
Union Forces Engaged: 133,868
Winner: Confederacy
Casualties: 30,099 (17,278 Union and 12,821 Confederate)
--------------------------------------------------------------------------------
#4
Battle of Spotsylvania
Date: May 8-19, 1864
Location: Virginia
Confederate Commander: Robert E. Lee
Union Commander: Ulysses S. Grant
Confederate Forces Engaged: 50,000
Union Forces Engaged: 83,000
Winner: Confederacy
Casualties: 27,399 (18,399 Union and 9)000 Confederate)
--------------------------------------------------------------------------------
#5
Battle of Antietam
Date: September 17, 1862
Location: Maryland
Confederate Commander: Robert E. Lee
Union Commander: George B. McClellan
Confederate Forces Engaged: 51,844
Union Forces Engaged: 75,316
Winner: Union
Casualties: 26,134 (12,410 Union and 13,724 Confederate)
--------------------------------------------------------------------------------
#6
Battle of The Wilderness
Date: May 5-7, 1864
Location: Virginia
Confederate Commander: Robert E. Lee
Union Commander: Ulysses S. Grant
Confederate Forces Engaged: 61,025
Union Forces Engaged: 101,895
Winner: Inconclusive
Casualties: 25,416 (17,666 Union and 7,750 Confederate)
--------------------------------------------------------------------------------
#7
Battle of Second Manassas
Date: August 29-30, 1862
Location: Virginia
Confederate Commander: Robert E. Lee
Union Commander: John Pope
Confederate Forces Engaged: 48,527
Union Forces Engaged: 75,696
Winner: Confederacy
Casualties: 25,251 (16,054 Union and 9,197 Confederate)
--------------------------------------------------------------------------------
#8
Battle of Stone's River
Date: December 31, 1862
Location: Tennessee
Confederate Commander: Braxton Bragg
Union Commander: William S. Rosecrans
Confederate Forces Engaged: 37,739
Union Forces Engaged: 41,400
Winner: Union
Casualties: 24,645 (12,906 Union and 11,739 Confederate)
--------------------------------------------------------------------------------
#9
Battle of Shiloh
Date: April 6-7, 1862
Location: Tennessee
Confederate Commander: Albert Sidney Johnston/ P. G. T. Beauregard
Union Commander: Ulysses S. Grant
Confederate Forces Engaged: 40,335
Union Forces Engaged: 62,682
Winner: Union
Casualties: 23,741 (13,047 Union and 10,694 Confederate)
--------------------------------------------------------------------------------
#10
Battle of Fort Donelson
Date: February 13-16, 1862
Location: Tennessee
Confederate Commander: John B. Floyd/Simon B. Buckner
Union Commander: Ulysses S. Grant
Confederate Forces Engaged: 21,000
Union Forces Engaged: 27,000
Winner: Union
Casualties: 19,455 (2,832 Union and 16,623 Confederate)
--------------------------------------------------------------------------------