No one answer
Unfortunately, there's no single value of
line-height (leading) that is optimal for all situations. An optimal range is probably roughly 1.3–1.7, but to select an optimal value requires we look at the specific font in use and the width of lines of text (among other things).
In Troy Templeman's excellent article Basic Rules of Good Typography, he says (emphasis mine):
Leading (or line-height) is the amount of vertical space between lines
of type. The default leading in page layout programs is usually
sufficient but there are a few factors that may require it to be
adjusted. A font may have long ascenders and descenders that touch
each other between lines of type, causing a distraction. If a font has
a high x-height, it reduces the negative space between lines of type
that gives the illusion of tight leading. Tight leading makes it
difficult for the reader to find the start of the following line of
type which is particularly noticeable in long lines of type.
Therefore, for wide columns of type, a generous line height results in
better readability. For body copy, I usually set the leading so that
the height of captital letters fit perfectly between lines of type,
which is around 1.5 times the font size.
Two fonts of the same size will not necessarily appear to be the same size. For text of mixed case, the apparent size of the font is heavily influenced by the x-height, as is clear in the following graphic:
When a font has a larger x-height, it appears to have more space between lines, so it won't need as much leading to read comfortably. (As you might guess, for text set in all caps, the cap height is what matters, not the x-height.)
The second aspect to consider is the width of lines of text, also know as the measure. As the eye moves left to right along a line of text, when it reaches the end of a line, it needs jump back to the beginning of the next line. As the line width gets longer, our eyes have more trouble correctly finding the beginning of the next line if the line height is tight. (That is, we make more mistakes such as starting to read the same line twice or skipping a line.)
Mark Boulton's article Five simple steps to better typography states:
Your leading should increase proportionally to your Measure. Small
Measure, less leading. Wide Measure, more leading. It’s a simple but
(If you're curious to read on, he later discusses how color choices also affect optimal line height.)
As you can see, it's impossible to choose a single optimal line height. Rather, you must primarily consider your font's x-height and the length of lines in your text. Be sure to read Alan Gilbertson's excellent answer to a very similar question on the Graphic Design Q&A site.