I wouldn't say Kobayashi's most dangerous. He's very aggressive, but just about all the contact he's been involved in has either been caused by the other driver or, as in Australia, the result of a mechanical failure.
It's hard to pick one dangerous driver, even for this year, because many of them have made ill-advised or just plain idiotic moves, but for most dangerous move I'd say Massa at Suzuka. The fact that he wasn't penalized for dive-bombing onto the grass and causing a major incident just proves that the FIA favors Ferrari. That is unless he had some mechanical failure, so correct me if this is the case.
If I had to pick one all-time, it's hard to go against Ide. It's hard to get your Super License revoked, let alone mid-season, and only after 4 races.