The most ambitious philosopher of the 19th century and the one whose influence is hardest to escape — you are either a Hegelian, an anti-Hegelian, or quietly using his vocabulary without noticing. Hegel's central claim is that reality is Spirit (Geist) coming progressively to self-consciousness through history, and that this unfolding has a logic — the dialectic — in which contradictions are not errors to be avoided but engines of development. Thesis and antithesis are aufgehoben (canceled, preserved, raised up) in a synthesis, which becomes the next thesis.
In political philosophy, this means freedom is not the absence of constraint (Negative Liberty) but the rational self-determination realized through the institutions of family, civil society, and state — what he called "concrete freedom." The state is not a necessary evil but the sphere where rational freedom becomes actual. Read charitably, this is a deep argument for constitutional order; read uncharitably (as liberals from Isaiah Berlin onward have read it), it slides toward making the state the judge of what its citizens really want.
Karl Marx famously claimed to have stood Hegel on his head — keeping the dialectic but making its driver material rather than spiritual. 20th-century "Western Marxism," existentialism, and phenomenology all worked through Hegel; so did Hannah Arendt and, more surprisingly, much of contemporary analytic philosophy of mind.