The best in my limited travels east of the Rockies: Antietam.
The National Park Service has maintained a laudably light-handed and especially reverent vigilance of the well preserved battlefield proper. It's quiet stillness, non-commercialized, reverent atmosphere is quite palpable.
Honorable mention should also go to Appomattox Court House (while not exactly a battlefield); it's beautifully preserved, non-commercialized grounds is surprisingly inspirational, whether you be a Southern sympathizer or misguided Northern interloper.
As for the worst . . . without question: The Alamo . . . surely, a much over-hyped, mostly disappointing, and far too commercially consuming of an experience. Find the spot where Davy Crockett and his brave men stood, defending the stockade, and surely shudder to witness what Davy, too, must have espied: a sprawling concrete plaza, facing a slough of downtown multi-story shops, across an otherwise bustling city street of non-stop autos. The otherwise good State of Texas sadly failed early on to take measures to officially recognize it's own original landmark, underfunded and overwhelmed - surrounded! not by an unstoppable mass of under-nourished Mexican soldiers, today, but rather by exceedingly cloying downtown San Antonio commercialism . . . oh, wait, wrong war.
hmmm . . . I must defer "the worst preserved" acw battlefield site to others who have traveled more than I - although I confess I was personally disappointed with the Gettysburg battlefield for reasons not so dissimilar to the kind of over-the-top crass commercialism that has intruded so heavy-handed upon the still forever poor and beleaguered ghosts of the Alamo; this, despite the otherwise truly astonishing "Gettysburg Cyclorama" (by French artist, Philippoteaux) and the many inspired monuments on the grounds proper.
Fd Lt shoeless
Secretary of the Cabinet, CSA (Retired)
1st Tenn Provisional Army
<center><i>From a certain point onward there is no turning back. That is the point that must be reached.</i> --F. Kafka</center>