Which noSQL database is best for high volume inserts / writes?

If you want fast write speed, you can just insert your data into memory and flush data to the disc at a background every minute or so. That should be fastest solution.

MongoDB and Redis do this actually. For example, in mongodb you can go without journal enabled and writes will be very fast. But keep in mind that if you store data in memory at a single server there is possibility to loose your data (data that not flushed to the disc yet) when your server goes down.

In general, what database to use highly depends on data you want to store and task you are trying to solve.

Apache Cassandra is great in write operations, thanks to its unique persistence model. Some claim that it writes about 20 times faster than it reads but I believe it's really dependent on your usage profile.

Read about it in their FAQ and in various blog posts.

That is, of course, if you have "classical" DB profile of large amounts of data. If your data is small, or is used temporarily and/or as a cache layer, then of course opt for Redis which has the fastest throughput both for reads and for writes, since it's memory-based (with eventual disk persistence).