I have been to 176 National Park Units. I am always curious which units people find the least interesting, and why. I take a couple things into consideration when pondering the above question.

How interesting was it to get there?

Is it off the beaten path? Getting there is just as interesting as the site itself sometimes.

How pretty are the surroundings? is it in a cool area of a cool city? (People say they don't like Castle Clinton, but it's in such a beautiful area that I really dig it).

Are there cool walking trails?

If it's someone's house, is it unique? Is there original furniture?

Is the subject matter something obscure? like will I learn something cool that day. (for example, Maggie L walker, Carter G Woodson, Horseshoe Bend, River Raisin, things that were not on my radar before visiting)

I am always satisfied tbh, but If I had to pick least interesting - here are my top 3 thus far.

1. Martin Van Buren NHS NY - right off a main road, he's not the most important president, and learning about his achievements didnt blow me away lol. Also the grounds are pretty small and modest.
2. MLK JR memorial DC - It's in a pretty area but the statue is pretty uninspired and the MLK unit in Atlanta is super compelling in every way, so I don't really get why they added this one. Perhaps it hits differently at night. I feel that FDR memorial and OKC Bombing memorial are both amazing at night, and less so during the day.
3. Hohokam Pima AZ - it is literally not there. It has been reburied by the tribe that owns the land. Something about that made it hilarious and interesting to me tbh, but alas, it is literally not there.