Morphia may be the most stable of the three. I have not heard much of Mongo4j lately- probably abandoned. I personally like spring-data because of the hades project... You don't need to implement the DAOs. You just write the interface and spring data automatically provides it to you. However Spring Data Mongodb implementation seems a little buggy in my initial trial. If you have hard dates and is working on a production quality product, probably it is wise to choose Morphia.
