Depends on what OS you want to run.
For the 5,1 Mac Pro
If you are running Monterey and don't want to do the Pixlas Mod then AMD Radeon RX6800 flashed for Mac
If you are willing to do the Pixlas Mod then the RX6800XT flashed for Mac
Now here is the stupid part if you need to upgrade even temporarily to Sequoia, in my case to run DFU for an upgrade on a Mac Mini M4 then you will need to downgrade your video card.
I had to put back in a GTX680 to be able to upgrade a drive to Sequoia
Yeah I know backwards right because anything after Monterey doesn't support AMD Radeon GPU
So if you are wanting run Sequoia and not do a Pixlas Mod definitely a RX580
For the 5,1 Mac Pro
If you are running Monterey and don't want to do the Pixlas Mod then AMD Radeon RX6800 flashed for Mac
If you are willing to do the Pixlas Mod then the RX6800XT flashed for Mac
Now here is the stupid part if you need to upgrade even temporarily to Sequoia, in my case to run DFU for an upgrade on a Mac Mini M4 then you will need to downgrade your video card.
I had to put back in a GTX680 to be able to upgrade a drive to Sequoia
Yeah I know backwards right because anything after Monterey doesn't support AMD Radeon GPU
So if you are wanting run Sequoia and not do a Pixlas Mod definitely a RX580