If we're still talking about American Wars the clearly with over 600,000 casualties, the Civil War was the bloodiest.
As for battles, that's a tougher one. Tarawa (WWII) has been mentioned. In it, 4,500 Japanese were killed (only 20 were left alive), 3,000 U.S. Marines were killed or wounded in 4 days of fighting.
In comparison, the Civil War's costliest battles were:
1. Gettysburg - 51,112 (23,049 Union and 28,063 Confederate)
2. Chickamauga - 34,624 (16,170 Union and 18,454 Confederate)
3. Chacellorsville - 30,099 (17,278 Union and 12,821 Confederate)
4. Spotsylvania - 27,399 (18,399 Union and 9,000 Confederate)
5. Antietam - 26,134 (12,410 Union and 13,724 Confederate)
These are followed in order by The Wilderness, Second Manassas, Stone's River, Shiloh, and Fort Donelson. Sorry but Sharpsburg didn't make the top 10 although you're right there were a staggering 23,100 casualties.
See
http://www.civilwarhome.com/Battles.htm for more details.