Saadia Gaon, Rambam, John Locke, Kant, Schopenhauer, Kelley Ross.

Mainly the Left is based loosely on Rousseau, Hegel, Freud, Marx, Nietzsche. Also it is highly connected with existentialism. These all seem to me to be wrong turns. It once was considered the most sensible. 

The better approach seems to me to be based on Saadia Gaon, Rambam, John Locke, Kant, Schopenhauer, Kelley Ross.

I perhaps should go into what is wrong with the Leftist philosophers but the critiques are well known.
I do not see what I could possibly add to the discussion except to say that the emphasis in learning and education ought to be shifted away from second rate philosophers to first rate philosophers. Why concentrate on what is wrong and flawed?
[To go into what is wrong with Nietzsche in short: moral values have prima facie validity. To defeat moral values you would have to have starting principles that have more initial plausibility. That is let's say you have a principle A that seems sort of OK. From A is implied B. Then if B makes no sense then you would have to reject A. All leftist philosophers start from some A that sounds sort of OK. They come up with something nice sounding but which has very little initial plausibility, but being a naive first year college student you really do not have the intellectual power to  disagree.  Then from A is deduced some B like all morality is relative. Since B has no initial plausibility it would require some strong A to prove it.  Just the opposite "not B" has more prima facie plausibility than A.] [What is wrong with Post Modernism is this: Frege wanted to expand the a priori. This was easily defeated. But then people took this defeat to mean there is no a priori. The problem was תפסת מרובה לא תפסת. Don't bit off more than you can chew. Do not try to prove too much.]