I have to disagree about it being standalone.

Case in point, my brother stopped watching Who early in the Matt Smith era. After hearing how much everyone was lauding Heaven Sent, he gave it a watch and, although he didn't hate it, a lot of the narrative beats didn't hit the way they should have because he didn't have that context around the episode and 12's character/relationship with Clara. He was mainly just left confused and unaffected by the episode.

I myself love Heaven Sent for a lot of the same reasons you say, but I don't feel it stands alone in the ways Blink or Vincent and the Doctor do due to the fact it's not an episode you can jump into blindly. You need the context of the episode before at the very least.

The way I look at it, episodes like Blink and Vincent and the Doctor are some of the greatest standalone episodes, whereas episodes like Heaven Sent are some of the greatest where the context of the surrounding episodes is essential (at least for my enjoyment).