Which hard drive is my best choice for an NVR?

Purple are designed for NVR so the answer is pretty obvious.

Usually, the NVR would include a list of supported devices, tested.

Thanks experts! You have given me enough information to make a good decision. I think I will buy the Purple 8TB drive. I will first check to make sure that the NVR will accept this updated current drive. They are not cheap though. It will cost about $370. 

which NVR, the size of the Drive will tell you how far back you can review the data, Do you need the extended duration for retention?

I.e. if you have 2TB now, and it meets the retention duration, why get 8TB. IF the 2TB barely met the retention requirements, why not get 4TB or 6TB.  Criteria for deciding the size of the drive. Do you need the additional space for the additional duration?

At present I have a 6TB purple drive. I am getting warnings that the drive has excessive hours on it and it should be replaced. Normally I operate 7 to 8 cameras that each record full time. Recently I only have 6 cameras in operation. With those 6 I am seeing about 5 weeks of storage of these videos. When I get back to my normal 7-8 I guess I will get about 4 weeks. For my application 4 weeks is the minimum I want to deal with. Naturally more is better. The reason I am leaning toward the 8TB is the statement that Jackie Man gave as "The 8TB-18TB models have different AllFrame AI firmware supporting 64 cameras and an additional 32 streams for deep learning analytics within the system to improve video streaming quality. This is for network video recorders (NVR) applications which encode and process data at the camera. ".  

Is it a single Drive NVR, or it has multiple drives and is recompensing that one of them should be replaced?
 

The features you reference, does your setup benefit from their presence?

It is a rebranded Hikvision 8 channel 4k unit. It can use 2 hard drives. It only has one in there at present. I have never used two. I am far from an Expert on NVR hardware. I am slowly learning the capabilities of my system. I was interrogating the system and saw a remark or advisory that was telling me that the drive had a great deal of hours on it and I should consider replacing it. I believe it was a generic message being generated by the NVR to be extra cautious. I was considering an option just to see how it might work. I have a spare 1TB Samsung SSD that I could install as the second drive. It is unknown to me how the NVR would utilize this drive. I think that if the drive were either incompatible or of little benefit that the NVR would notify me of this. Do you think it is something I should experiment with? Oh, and I wanted to thank you for the additional interest in my situation. Much appreciated!

If not mistaken, it will use the combined space. It may offer a way to designate only one drive  as the sole destination to allow for the slow wean before the old drive is phased out, replaced.

which model DS-7608NII? Checking the manual to make sure the addition of a second drive will not impact the information on the new one and how it handles ....

In short, that would be a path to try.