Selecting frames for stacking...

Selecting only the best frames to use seems to be the most important
factor in producing quality stacked images. Most stacking applications
are able to rank frames by some "quality" criteria, helping to
manually winnow the poor ones.

But which software (freeware, commercial or otherwise) does this best;
either automatically or by offering fine control over the quality
ranking process?

Is there any stacking software that uses partial frames (when some of
a frame is distorted it appears that other parts could still be used
if the distorted bit were masked out).

Cheers
Beats