What are the most significant contributions Slavoj Žižek has made to the field of philosophy, perhaps in particular to political philosophy?
Although Hegel was always important, especially due to Marx, he seems to have given a freshly challenging Hegel to the world. Also, surely, the Slovenian School, has all but appropriated the body of Lacan, and distorted the previous reading to the point of an alienation, that is now applied by practicing analysts.
If you know how to do a search of the journals, in fields such as sociology and ethnography I think you will find it is not unmoved by his inquiry. The style of reading alongside the actual transformations and revivals along with the doctrine of transcendental materialism, as a great move from the primitive Marxist position amount to a synthesis of Marxism and Hegel such as Marx himself never achieved.
By this I mean that Marx ignored Hegel's great and lasting insight that wisdom comes only with the dusk, claiming that, hither to the philosophers have only interpreted the world (the task is to change it), the discovery of Hegel was that this is impossible in the proper sense. However, Hegel could not finish his own objective work, since he did not live in this day, but his, he had in mind certain rational laws, but in order to delineate them one must live to see the way the society and culture unfold and make themselves know in actual terms and so Zizek as attempted such work. Perhaps very badly, but who knows?
Unfortunately, Žižek is misinterpreted and misunderstood very often. He is famous but this fame is rather only bringing a demise to his reputation and work. This is why most people consider him only the "Borat of Philosophy", and not a real philosopher. Furthermore, academia widely rejects him. It is because of these reasons that the true value of his work is not yet known and underestimated. Perhaps it is going to take few decades until we truly understand what huge contributions he has made to the field of philosophy [and psychoanalysis].
One of the most important known contributions of Žižek is the revival of dialectical materialism. I cannot recommend his extremely important and fascinating work Less Than Nothing: Hegel and the Shadow of Dialectical Materialism enough.
You are in particular asking about 'political philosophy' and that would be perhaps even harder to answer. In the 2005 documentary directed by Astra Taylor named "Zizek!, he himself mentions that he never truly enjoyed writing his political books. Nonetheless, he has offered great and unique insights into politics over the years. You can look into his book "Welcome to the Desert of the Real" as a start if you are in particular looking for his political writings.
The best way to explore Zizek is by reading his works and even watching his lectures/talks on YouTube, but I highly recommend you to read his books first and definitely watch "The Pervert's Guide to Ideology (2012)" and "The Pervert's Guide to Cinema (2006)".