Requiem is arguably the saddest.
I'm currently running through the entire DEH original broadway soundtrack, and I honestly believe Requiem is sadder and more heart-wrenching than even Words Fail (its close though) and thats mostly because of the idea of how much Evan's lies really affected connors family as they grieve.
Of course initially as they grieve connor they are searching for a reason and something to hold on. Evan's lie allows Cynthia to hold on to Connors memory through the idea that he existed in a way that she never got to see. For Zoxe and Larry, they believe he lived in a way they never got to see, but they understand it as he may have hated them so much he believed they didnt deserve to see that side of him. Zoe and Larry, essentially lie to themselves, saying if he didnt care why should they? But they are still grieving, and still can't accept the idea that Connor would do that, even if it was subconscious.
The portrayal of grief has always had a major impact on me. And for that reason I think Requeim is the saddest song. I dont even know if thats an unpopular opinion, I just really wanted to talk about this and thought yelling it into the void that is the internet would be a good solution lol