If we're talking about everything as a whole and I were to select which aspect I love most about the movie, I'd probably say Octavia Spencer's performance as the titular Ma. She was absolutely freaking great as she nailed every acting beat that she needed so perfectly. Any time I felt the sudden shock of a fear surge through my body, it was largely due to Spencer in how she was able to shift the look in her eyes from the chill and happy to a cold anger where I wasn't sure what she would do next. She was my favorite part.
If we're talking parts, as in scenes or plot points of the movie, my favorite part was honestly the ending. I don't want to spoil anything for anyone who hasn't seen it, but how it ends with Octavia's character smiling in that bed... I was enamored by everything going on and I thought it ended on just the right note.