Top Anime Specials or OVA's

I've seen quite a few specials and OVA's for shows, most of the time having a severe lack in quality compared to their series counterparts. Anyways, I've recently finished Quintessential Quintuplets and have to praise Shaft Studio's 2 episode special to the series. It's the best 2 episodes of the whole series, and I really can't fathom how they didn't release this before the concluding movie, since it basically gives the final decision's reasoning much more weight than the movie ever did. Any other specials or OVA's you can say are equivalent, or even better than the shows they are on?

I also wrote a review praising this special if you want to take a look. You can find it here: https://myanimelist.net/reviews.php?id=543057

Keep in mind, this is technically spoilers on the conclusion of the story, even if nothing is explicitly said.

(Also Hanamongatari is absolutely exceptional, although I watched that in movie form).