Best Data Store for huge data with large number of reads and writes
Based on the characteristics you've mentioned (JSON Documents, accesses by key, 100 million records, balanced read/write) I'd say CouchDB or Membase are good candidates (here's a quick comparison)
Both HBase and Cassandra can probably also work but for HBase you'd need to install a lot of components (Hadoop, ZooKeeper etc) that you won't really use d only use and Cassandra is better when you have more writes than read (at least the last time I used it).
Big Table, is unfortunately internal to google : )