!rant
If you had to build an event based analytics system what database would you prefer? I was thinking mongodb. What would you use and why?

Probably Postgres. I know this is used heavily in analytics systems.

MongoDB is a good choice, considering you are gonna do a lot of reads compared to writes. Also the aggregate framework in Mongo is pretty great.

With that said, for very large data sets, you should probably look into something like Cassandra.

EDIT: Mongo Oplog is super helpful for event based system.