Hard choice...
If I had to chose an absolute favorite, I'd probably go with the first one I ever read, "Texans in Gray: A Regimental History of the Civil War" which is mostly a reprint under a different name of the memoir of Sgt. John C. Porter of the 18th Texas Infantry. (The book I think is VERY misnamed, still good book.)
Now if we're talking a tie of different books, "Footprints of a Regiment: A Recollection of the 1st Georgia Regulars" by W.H. Andrews is a favorite, and "Civil War in the Southwest: Recollections of the Sibley Brigade" is an awesome collection of memories from several soldiers who served in the New Mexico Campaign. Another favorite of mine is "Memoirs of Service Afloat" by Raphael Semmes, (except for the politics).
"A Creek Warrior for the Confederacy" is another awesome memoir with valuable information from before, during, and after the war, and is a decent read, and if we look beyond the CW the memoirs of Joseph Plumb Martin in the Revolutionary War is a great read.
I'm just gonna leave it at that, I've read and enjoyed too many to make a choice.