RethinkDB is built to store JSON documents, and scale to multiple machines with very little effort. It has a pleasant query language that supports really useful queries like table joins and group by, and is easy to setup and learn.
Ranked in these QuestionsQuestion Ranking
Pro Horizontal scalability
RethinkDB is scalable horizontally. It has support for sharding, parallel queries and MVCC concurrency.
Pro Changefeeds (change listeners)
You can listen to changes and trigger code based on these changes.
Pro Powerful query language
Pro Easy install
Only takes about 30 seconds to install. They also have a docker file for running it on AWS, Google Cloud or your own.
Pro Ease of cluster setup
You can directly tell it to shard/replicate and how many shards/replicas depending on the amount of nodes. Each node doesn't need anything except one other node's ip/port in the cluster to join and maybe the auth.
Pro CLI and web administration tools
RethinkDB has administration tools in both CLI and GUI (web app). You can view whats going on right away by going to localhost:8080. The data explorer allows you to run queries on the db.
Pro Auto master promotion
Unlike a lot of other databases where if the master is down the system is down, this one if the master is down someone else is made master so much more peer to peer.
Con Cannot run queries from its CLI
Con No user accounts
This is just the database, you need to setup your own auth and user accounts (such as using Auth0).