In MongoDB 3.0, MMAP (default) and WiredTiger are the stable storage engines. Usually, if your app is read-heavy, use MMAP. If its write-heavy, use WiredTiger.
Your solution may also have a mixed replica set members where you can have one node configured with MMAP and another with WiredTiger. You can use one to insert massive data and the other to read with analytical tools.
After MongoDB 3.2, WiredTiger becomes the default engine.