Do the primaries usually lead to the nomination of the best available candidates? why or why not

This is a complex question with the short answer being that they can but often do not.

Primaries, by their nature, put politicians through the ringer and make it so that their ideas are vetted by different types of citizens as they go along.

Our primary system has some flaws though, primarily that the early primaries are dominated by rural states.

So, candidates who may appeal to more citizens are often knocked out early because they may not appeal to white voters in New Hampshire and Iowa.

The counter to this argument is that the candidate that makes it through is the best candidate simply because they made it through the gauntlet of the American people. As a result, they represent a broad coalition.