CoC works very well as a OneShot system. Paranoia works, but is probably not something you would want to do as an unexperienced DM. I have had great experiences with running FATE oneshots without basically any preperation beforehands, but that requires improvisation skills and a group that just goes with it. If you have a real life group, i heard Dread is pretty cool, but have never played it. Also it is not something you can play on Roll20 as it involves a Jenga Tower.