What is the best color space for a digital photo to be displayed on an HDTV?
Well, really the best color space to convert your digital photo into in order to be displayed the best on an HDTV - is the custom display profile that you made of the HDTV.
But my guess is that you’re actually asking for what kind of generic profile you could convert to that might be close?
It looks like most modern HD formats call for Rec. 709 primary colors. I hunted around and found some test profiles we made on an HP Dreamcolor display that allows the tuning of individual RGB LED’s. So we made a profile of this monitor tuned to the Rec. 709 specifications. Then I compared this profile to some of the common working space profiles you might be able to get your hands on. And it looks like ColorMatch RGB.icc would be the closest to the Rec. 709 profile.
So you could try ColorMatch. But it’s probably a lot like using sRGB to represent the average CRT. (Your mileage may vary.)