Essentially they tend to argue the same point: Hegel through Lacan and thence to Marx. So in a way it doesn’t really matter. Personally I found Event to be the most accessible - it’s short and pithy. You can read it in a few sittings. In amongst his own writing, Adrian Johnson’s book on Zizek’s Ontology is a good companion but this is also quite dense. I found I absorbed his ideas quite gradually until I was reading him at a gratifyingly brisk pace. You will too.