I know this won't be a popular response but......actually none. 1080P 2K Blu Ray is as close to "perfect" as I intend to go. I've double dipped from DVD to Blu Ray on a lot of titles but I'll NOT be triple dipping on any title.
So shoot me!!
I will not be double dipping past a standard Blu Ray. The more sterile the look past a certain point is simply too much for me. My Blu Rays look good enough for me now. No more upgrading except the projector.