best video editing external hd for late 2012 imac

If I/O is critical and I would think it would be then consider a Thunderbolt equipped enclosure. There is not "best" at anything that is why you have choices available to you. What you are looking for is what comes closest to fitting all of your needs.

I'd recommend looking at OWC (www.macsales.com) they have the complete product selection. You can also use them for advise which product comes closest to fitting your needs.

SSDs will always be more reliable and quicker than a HD so if money isn't an issue go with a large external SSD connected via Thunderbolt.