Personally, I think we are all in love and fascinated by the large scale battles - Gettysburg, Antietam, Wilderness, Shiloh, and Chickamauga.
But what smaller scale action are you fascinated by? For me, it would be a toss up between Ball's Bluff, the battles that comprised Burnside's North Carolina Expedition, and the September 1863 battle of Sabine Pass in Texas.