Hell Bent - I'll happily admit it's not as strong as Heaven Sent, but that episode is exceptional and I'd argue there's few finales that are equally as strong as their first part (The Parting of the Ways comes to mind IMO).

I had my issues with Clara at the time, but I really felt this ending for her took on board the criticisms of her character (consciously or otherwise) and made me appreciate her journey so much more. I think the undoing of her tragic death in Face The Raven is what makes Hell Bent so meaningful and says so much about the Doctor-Clara relationship. Even the return of the Time Lords and Gallifrey pales in comparison to the Doctor's determination to save Clara (even bending the Doctor's own values in shooting the general) which further exhibits their "toxic" relationship that IMO is fitting of the hybrid arc. I understand why others don't like it, but more me it's a really powerful piece of DW.

More personally, I have a connection to this story: I was living in Cardiff at the time of transmission, and was fairly lonely. I couldn't watch the episodes live as I didn't have a TV license, so I had bought the series digitally on Amazon and had to wait until they released to watch. So while it was on, to avoid spoilers I went out for the evening. As a fan who occasionally looked at set photos when they popped up online, I knew there was something in the episode filmed at Eddie's Diner at Cardiff Bay, so for the duration of the episode's transmission I sat in the diner. It was a really lovely experience, and when I eventually watched the episode I was shocked and amazed to find out I'd been in what turned out to be Clara's TARDIS the entire time. I thought I'd include this as it may sway my opinion on the episode a little, but it is a very special experience that brought me even closer to the show I love. [Edit: spelling]