I have a 2TB Seagate GoFlex Desk External HDD. I want to format it. Which is the best disk format, which will be supported in all OS, like Windows, Linux and Mac?
I've found NTFS, sadly, the be the "most" portable.
In most cases you'll at least be able to read ntfs without OS modifications.
For write support in Linux, you may need to install ntfs-3g.
You could use something like ext3 which anything calling itself a unix derivative should support quite well. You can install this to get support in Windows. That driver will only mount the ext3 file system as an ext2, which will work, but disables file system journaling, meaning you'd have to do some scanning/fixing if you crash the file system or uncleanly mount it in some way.
OSX supports ext3 out of the box (which it should, being a unix system). But you may have problems with Finder automounting it, probably depends on the version of OSX. Look here for some mounting tips, if necessary.
File Server, if you don't care about the external drive part
If you want to use that drive to store massive amounts of data for various machines, you may want to considering running a file server for it. Some people get all whiney about incurring extra power costs, so you can consider that, though if you do it well the cost won't be a lot.
You could find a computer to toss the drive in, set it up with FreeNAS and provide access to your drive over your network using the samba/cifs file sharing protocol - which all major OSes support very well.