Most answers here are pretty old.
In the past when un managed nand was the most popular storage technology, yaffs2 was the most common file system. This days there are few devices using un-managed nand, and those still in use are slowly migrating to ubifs.
Today most common storage is emmc (managed nand), for such devices ext4 is far more popular, but, this file system is slowly clears its way for f2fs (flash friendly fs).
Edit: f2fs will probably won't make it as the common fs for flash devices (including android)