It depends in part on the size files you'll be transferring.I have a WD Elements 500GB drive. I will be using it to transfer files from a PC (Windows7) to my iMac (SnowLeopard). What would be the optimum file system i should format the drive in order to do this without problems?
I recommend you to use exFat.
Those Mac OS X NTFS emulators didn't work 100% for me and I had some issues while testing.
FAT32 has 3 or 4 GB file limit. ExFat does not - and it works fine for me.