A-day for sure. That's of course when your Plebe has completed 6-weeks of Cadet Basic Training and is "accepted" into the Corps of Cadets, going from a "New Cadet" to "Cadet." They get get off post privileges but must be back in the barracks in the evening. If you have to pick, you certainly want A-day over R-day.
A relatively new event is the Parents Weekend in October. Last year and this year, it has centered around a home football game on Columbus Day weekend, a three-day weekend for the Cadets. They can take a pass and spend the night with you in your hotel. They have tours available of the barracks, mess hall, the Supe's house and the uniform factory. You can eat in the mess hall with your cadet on Friday IIRC. It can be crowded though as it is a football weekend and this event is for ALL parents, not just plebes.
Finally, there is the Plebe Parent Weekend in March. The upperclassmen are released on Friday and there is a hop for the Plebes that the parents can attend also. Again, tours of Supe's house and uniform factory. Only the parents of plebes are usually there and the plebes are the only cadets there so they feel more relaxed. Or so I've been told.
Those are the four big parent events during plebe year.
I'd rank them
1) A-day, a must, don't miss
2) Plebe Parent Weekend - I haven't been yet but I think seeing the hop would be neat and a lesser crowd would be really nice compared to A-day
3) R-day - you drop your new cadet off in the AM after the 60/90 second goodbye and you can tour around after that but no more close contact with the cadet.
4) Parents Weekend - could be higher, a home Army football game is something to experience.