I voted LangChain because it's the only one I've used. The documentation is great, tons of out of the box packages and integrations, lots of good examples, seems to be the most common, and LangGraph is also very nice.
LlamaIndex seems focused on RAG.
For my job, I built a SQLiteVec RAG app using LangChain and their SQLiteVec vectorstore class (which I modified to allow for metadata pre-filtering). I also used LangGraph to build a workflow for the Chat interface on
Staqc.com, and it's really nice that they have a draw method which outputs a mermaid diagram of your workflow/agent system.