A loaded question, I know.
You got that right.
But if you had to pick one color film for best image quality, which one would it be, and why? Please support your answer with real-world experience, RMS granularity numbers, D-Max/dynamic range figures, resolution numbers, and/or scientific tests.
It depends of course on what you are trying to accomplish. Negative films and trannies are really different, with strengths and weaknesses that are different enough that comparisons are nearly meaningless.
RMS granularity numbers can generally be obtained from the manufacturers. What these numbers don't tell you is the range of dye cloud sizes, and therefore have little meaning if you are trying to figure out which film will be "more grainy" than another.
Dmax is probably more interesting for tranny films. With negative films it's considerably more variable.
Dynamic range is hard to quantify. Tranny films have less than negative films in general, but how much is useful in negative films is difficult to determine because the harder you work the film, the more likely are color shifts and crossovers. There's no hard and fast line in the sand here -- depends on what you can live with.
Resolution numbers are way more interesting when you include the entire system -- that is, nearly all photographic resolution is limited by something other than the film.
This leaves you with real-world experience which, if you get enough answers, reduces down to a popularity contest.
The bottom line is you're going to have to do some testing and experiments yourself, with your own photography, and your own workflow. What works for others might be interesting, but may not pertain to your personal situation. The only way to find out what will work best for you, is for you to do some testing yourself.
All that said, personally I use 160PortraVC for my work. I shoot a fair amount of scenes with large subject brightness ranges so I want the large dynamic range (I've recorded 11 stops for some of my images, without any noticeable color shifts or crossovers). The negative film gives me excellent color fidelity also (that's in part what the orange mask is doing). Finally, I'm shooting 5x4, so graininess really isn't an issue for me. Sharpness is however, and 160Portra is surprisingly sharp. Nearly as sharp as TMY-2, which tells me that I'm probably optics limited (OK, OK, technique limited, I'll admit it!) and not film limited. But mostly I like the pallet. It handles a wide range human skin well, under a range of lighting conditions, and it also renders landscape well. A nicely well rounded film IMHO.