For me it's Tainted's beautiful Eadie. Ally and Angus, though I didn't follow her original thread on here, I followed her story elsewhere and have kept up with her recent threads here. Sian's mum, that devastated me and caused me to change my username so I think about her often.
Also, London's shameful secrets of motherhood thread. One of the best threads ever and I love seeing BH put judgement aside and support each other on things that would be torn to shreds in other threads but that one just feels like a safe space.