It really depends on the camera. And personal taste. The only way to know is to try for yourself in a controlled environment and then see what results you get in the field with the settings you've chosen... adjust until you're happy.
With the 5D3, I find the read noise is really gnarly but the gain is pretty clean. So I turn HTP off and try to shoot at 100, 160, 200, 320, 640, or 1280 ISO when possible. Beyond that it gets pretty noisy. Sometimes. I've used the camera up to 10,000 ISO and found the video acceptable, though. It really depends. The shadows are grainy, but the rest is pretty clean so if you keep your exposures good and don't mind losing some shadow detail in post you can get away with anything.
If you can light with very low contrast and expose properly at 160 ISO that will probably give you the best results, but I'd say up to 640, even 1280 is pretty safe if you keep HTP off and don't use cinestyle, which lifts the blacks too much and makes exposing difficult. Dynamic range should be pretty similar, though HTP will lose you a stop in the highs for cleaner mids and shadows.
For red, Alexa, and the canon cinema cameras, 800-850 ISO is the normal iso, but you can pull the image digitally and make it cleaner. I just saw some Alexa footage shot at 400ISO. Such great tonality. But the trade off is you need more light and highlights clip sooner.
I wouldn't sweat it too much. Just keep your ISO low if you can and avoid HTP, cinestyle, etc. "Professional" video is shot all over the place. (As are stills.)