what is the best external hard disk format to be used for both MAC and Window

Hello good people,

I have an external hard disk and it's giving me trouble. Sometimes it takes a long time to mount on Mac and other times it refuses to open on my Windows partition on my iMac. But the hard disk works TOTALLY FINE with my HP Laptop. So I'm guessing the problem is from my iMac some how.

Anyway, what I'm wondering is, what is the best file format to format my external hard disk to, to be able to perform fast and steady in both Mac and my Windows on the boot camp?. Fat32, or NTFS, or what?.

I hope you guys can help ^ ^