This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Last Five Years
"Still Hurting" is the ultimate breakup song.
"Goodbye Until Tomorrow/I Could Never Rescue You". Even though you already know everything that has happened at this point, the juxtaposition of Cathy's excitement and Jamie's miserable resignation is just HEARTBREAKING.
There's a line in "The Next Ten Minutes" that, once the Fridge Brilliance kicks in, is almost just as heartbreaking. During the wedding song, Cathy and Jamie sing about being together "Till there's no one left / Who has ever known us apart". During the show, because of the way the two timelines work, they are only actually together onstage twice—during their wedding and at the ending of the show when their timelines finally separate. And that reflects how they were never emotionally together in the first place…
"Nobody Needs to Know". It's the regret Jamie feels, and the Dark Reprise of "I could be in love with someone like you". Damn Norbert Leo Butz and the break in his voice as he does the line "All right, everyone bleeds! All right, I get what I need!".
"See I'm Smiling":
The way Cathy tries so hard to be what Jamie wants and not rock the boat. Followed by her epic breakdown when she just unloads everything that's wrong with their relationship.
Cathy: I swear to God, I'll never understand, how you can just stand there straight and tall… and see I'm crying… and not do anything at all.
Terrible, terrible Fridge Brilliance: in the middle of her rant to Jaime, Cathy sings the following:
Cathy: And the point is, Jamie,/that you can't spend a single day that's not about/you and you and nothing but you,/"Mahvelous" novelist, you!/Isn't he wonderful? Just twenty-eight!/The savior of writing!/You, and you, and nothing but you!/Miles and piles of you!/Pushing through windows and bursting through walls/En route to the sky!/And I...
From the movie version, especially if you've ever been cheated on or betrayed by a former significant other.
Jamie: Cathy, you're being crazy! Cathy:[visibly in tears] No, I'm not, NO, I'M NOT!
During her penultimate song to Jamie, "I Can Do Better Than That" (which is chronologically taking place at the very beginning of their relationship, but is at the end of the show, after the previous "you and you and nothing but you"), she sings the following:
Cathy: I want you and you and nothing but you Miles and piles of you Finally I'll have something worthwhile To think about each morning You and you and nothing but you No substitution will do Nothing but fresh, undiluted and pure Top of the line and totally mine!
She mentions how disappointed she was that her last boyfriend left her with nothing but a "heartfelt letter". The fact that Cathy's pure, unadulterated love for Jamie has been so twisted by the end of their relationship and that she sees none of it coming is heartbreaking enough, but what's worse is that Jamie's final song, "I Could Never Rescue You", is him writing a 'heartfelt' (whether this is true is debatable) letter to inform Cathy that he is leaving her after 5 years of being together. Poor, poor Cathy. * sob*
Every single time Jamie and Cathy express how happy they are in their relationship ("Shiksa Goddess" and "A Part Of That!"), because you know from the first fucking line in the show that it's all gonna crumble.
"A Part of That" is perfectly fine until the last bit of the song. Something about the way Sherie Rene Scott sings, "I'm a part of that, aren't I?" seems to bring up a lot of self-doubt, which is then confirmed when we realise just how emotionally distant Jamie and Cathy are from each other.
Once you've heard "I Could Never Rescue You", you can never revisit the show the same way again. Hearing that wistful, bittersweet waltz melody is bad enough at the beginning of the show once you know what it portends, but then hearing it again at the end of "The Next Ten Minutes" is an absolute gut punch when you know that this is the theme of Jamie giving up on the marriage for good.
"A Summer in Ohio" is an upbeat song where Cathy says how much she hates working in Summer Stock in Ohio, but how she's also happy to be doing it believing her life is finally moving forward and this is just a stepping stone to future success ("No, it's not nirvana but it's on the way!"). The song becomes a lot more depressing when you realize years later she's still doing the same thing.
In the movie, at the end of her first song Cathy takes off both her wedding ring and also a bracelet and watch. This makes when the bracelet and watch reappear (as gifts from Jamie) a sort of retroactive Tear Jerker.
Rewatching the show/movie proves just how little they actually understood each other in the end. Cathy's first song has the line "Jamie's convinced that the problems are mine/Jamie is probably feeling just fine/And I'm still hurting". During "I Could Never Rescue You", Jamie on the other hand confesses: "I'm not the only one who's hurting here/I don't know what the hell is left to do", proving that he actually is in pain as well and that he would like to deal with the problems, but doesn't know how other than to break up.