It matters what file systems you or your distributor build the Linux Kernel to support. Generally EXT2, EXT3, and EXT4 are the defaults. EXT4 is recommend for modern Linux installs. Many other files systems are be supported if built into the kernel FAT (12, 16, and 32), XFS, NTFS (using fuse), and etc.
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There is no default. Individual distros can choose their own default (usually ext3/ext4) but the kernel itself does not specify a default.