The existentialists[1] made some of the most thoughtful treatments of the human condition (among them, my favorite sub-group is the absurdists, who regard the human condition as absurd). Existentialism, however, offers a rather naive view of humanity, as possessing an inherent essence (they were essentialists), like reason, or disparate personal essences to which one can and should be "authentic". Nevertheless, their texts serve as the basis for much of modern philosophy (yet they are very entertaining).

[1]: http://en.wikipedia.org/wiki/Existentialism

[2]: More modern philosophers, like Michel Foucault, while accepting the premise of an absurd existence, reject the notion of an inner human essence, and firmly place the human condition within spheres of influence which they called "power", which then served as the basis for the modern definition of politics as the theory (and practice) of power.