fat32

Assuming your usb drive is /dev/da0

Using fdisk:

• Zero disk beginning with
  dd if=/dev/zero of=/dev/da0 bs=2m count=1
• Change first partition type to 12 (FAT32)
  fdisk -i /dev/da0
• Initialize fat32 file system
  newfs_msdos -F32 /dev/da0s1

Using gpart: