One and Only plus Forever and Ever but in reverse.
I just now finished One and Only. However I watched Forever and Ever (the sequel with a happy ending) first, then went back and watched One and Only. That's the only way I could handle the sad ending. I would not have watched it at all except for how much I loved the main's relationship in Forever and Ever.
I just had to see Sheng Chen and Shi Yi's story from the beginning. Normally I do not watch sad ending dramas, but I made an exception. It was worth it. I was on the edge of my seat for a good portion of the series, even though the beginning is slow.
Forever and Ever is a modern sequel, but it was interesting to see that it had elements from long ago woven into the story. Like the family home being the same type of historical setting, and some of the character's roles reminded me of similar roles in historical dramas. And of course there are many references to their first life in One and Only.