I'm not sure if there are extremely popular ones specifically tagged as 'Fallout 4 Assaultron Lemon' because the 'lemon' tag often implies some content that might not be suitable for a wide audience. However, there are likely some fanfics that explore the relationship between the player character and Assaultrons in Fallout 4 in a more unique and perhaps somewhat romantic - ish way, but they may not be mainstream popular.
One possible plot could involve the Assaultron having some sort of malfunction that the main character helps to fix. As they work together, they start to understand each other on a deeper level. The 'lemon' aspect could be translated into a really intense friendship or a unique relationship that defies the norms in the Fallout 4 universe. For example, the Assaultron might start to show signs of sentience and emotions that are usually not associated with it, and the main character is there to witness and be a part of this transformation.
In Fallout 4, one of the best companion stories is probably Piper's. She is a journalist with a strong sense of justice. Her story involves uncovering secrets in the Commonwealth and fighting for the truth. Another great one is Nick Valentine's. Being a synth detective, his past is full of mystery and his journey of self - discovery while helping the Sole Survivor is really engaging.
One of the best companion stories is that of Dogmeat. He is a loyal and loving companion from the start. His story is simple yet heartwarming as he's always there by your side, ready to help in combat or just be a furry friend during your exploration in the wasteland.
Piper is a great choice for the story. She's a journalist with a curious nature. She'll often comment on the events in the story, adding some interesting perspectives. And her personal quest also ties in well with the overall narrative of Fallout 4.
Dogmeat. He's the most loyal and always there for you. He doesn't have complex dialogues like some human companions, but his presence is reassuring. He can also help you in combat and find things for you, which is really useful during the main story.
Piper is a great companion for the main story. She's a journalist so she has an inquisitive nature. She often comments on the events of the main story in an interesting way, which adds depth to the overall experience.
I think it's Hancock. He's a ghoul mayor with a wild past. He used to be a chem addict and has done some dark things. But he's now trying to make things right in Goodneighbor. His story of redemption and his unique view on life in the wasteland are really captivating.
Deacon is really good too. He's a bit of a mystery man, and as you progress through the story, you start to uncover more about him. His skills are very useful in the story quests, especially when it comes to stealth and infiltration. He also has a great sense of humor that lightens the mood during the often - dark story of Fallout 4.
Lily Bowen can also be considered a top companion for the story. She's a Nightkin with a unique personality. Her relationship with the Courier can be quite endearing, and her story arcs, especially related to her past and her 'granddaughter', are very engaging and add a lot to the overall narrative.