All depends on what you are looking for in a battlefield visit. Gettysburg is the best marked and has the most information/comfort, Antietam is the most pristine/primitive to the time of the battle and Harpers Ferry is the most restored. If you only have a couple of days and are a journeyman historian on the Civil War, Gburg is your best bet. The visitor center and available multi media (movies, books, cassettes, dvd's, tour guides and so on) is the best. It will be almost impossible to see everything in Gburg in 2 days, I have been going to the field since 1961 and still have not seen everything. The recent clearing of the primary fields adds to the understanding of the actual events of the battle. Guides are good but there are varying degrees in what they present. My favorite books for Gburg walking tours that I have done are the Military College Tour Guide, becasue it uses the actual official records, Coddington's, Study in Command becasue of the approach to 19th tactics, Frassanito books because they will show the now and then images, Bundy Gettysburg Papers because of the insight and then it is just topical, like Pfanz on the second day fights, regimental histories and so on. Antietam is another story, my favorites are T Ballard Staff Ride, Military College Tour Guide, Gleam of Bayonets , Landscape Turned Red, regimental histories and so on.
This is just my opinion. Have fun with it and enjoy your time there.