The end of "The Honeymoon," with Callie intentionally shoplifting food from a convenience store in order to get arrested. The utter lack of hope in her eyes as she eats a candy bar while crying is devastating.
Callie and Jude's biological dad signing away his parental rights, so they can be adopted by Stef and Lena, even though he wanted to be able to be their father again, because it was the right thing to do.
"Padre" features flashbacks to the night Callie's mother died, and it's filled with heartbreak. From Callie recalling her Parting-Words Regret to her mother, nearly having a breakdown after misplacing her mother's necklace, which triggers repressed memories of how she destroyed her room in a screaming fit after learning about the accident, to finally breaking down in Stef's arms about her grief and guilt.
Callie: I should've kissed her goodbye!
"Adoption Day" gives us a slightly less Tear Jerker scene when it's revealed that Callie's biological father is someone other than she thought, meaning she can't be adopted until it gets sorted out. It's mitigated by a pep talk by Rita to not despair, Stef's assurance to Callie that they're not going to give up on her getting adopted, and Jude getting adopted (even though he said he didn't want to if Callie wasn't).
In 3x05, Going South, AJ goes off on Mike about how hard it is to be African-American as well as a foster kid. They had just gotten back from AJ getting racially profiled by one of Mike's partners.
AJ: You don't know how to help me, Mike! You got no idea what it's like to be living down to the low expectations society has for you! "Foster kid? Black kid? Oh, you must be headed for gangs, drugs, jail!" I've been hearing it so long, I'm starting to wonder if they're right!
AJ's grandmother in the retirement home. The poor guy can only downplay how hard it is for someone so close to you not even recognize you. His and Ty's reaction to her passing is also heartbreaking.
3x16, EQ:
It ends with Chloe, Rita's daughter, overdosing on pills. Chloe's dying body covered in her own saliva, in conjunction with the look on Callie's face, is especially painful. The ambulance manages to rush her to the ER in time, but Rita is absolutely devastated that she allowed her daughter to get this low, especially considering she'd cried for help a number of times before. This is the first and only time we see Rita this emotional, and it hits as hard as you would expect.
Rita: I never thought she'd actually do it.
Chloe's voicemail kicks off the scene in a way that's especially breaking.
When Lena loses her baby. The emotional reaction from her and the rest of the family is just heartbreaking.
In 4x01, Potential Energy, Mariana reuniting with the other family members after the possible school shooting with Nick.
In 5x01 'Resist', Stef's search for Callie, including whispering 'Callie' as she knocks on the trunks of cars in a parking lot where she believes Callie has been taken.
In 5x19 'Many Roads' there are a few moments, including Grace's death and Brandon's reaction, including when Callie finds him. To the opposite, watching each of the Adams-Foster kids graduate in succession.
