Alright, let's get this straight here. If you look at the title, it says "most dangerous...". This would imply that harm would come to us, thus making it dangerous. A black hole on the other edge of the galaxy poses no threat to us, so it is completely NOT dangerous.
A few years ago, a very large and dangerous meteor was headed for an area near the earth, and there was some concern about that. But out around Jupiter, the meteor fragmented and was sucked in by Jupiter due to its incredible gravity. The earth is pretty well protected from astronomical anomalies due to its inner location.
Because non-earth objects have a near impossible danger level, we turn to light-related anomalies. Gamma ray bursts, though fatal to the earth and the most dangerous in caliber, are very rare, and even when they do happen, there is a very slim chance that any of it will hit the earth. Therefore, it is not a contender for the title.
The sun, however, is essentially a nuclear power plant of gas resting 8 lightminutes from the earth. It has a very strong magnetic field, but sometimes that field fluctuates. This creates sun spots an mass ejections. Given a big enough fluctuation in the Sun's gravity, it can create very large mass ejection, which has been perceived to ruin electronic systems. Given a big enough mass ejection, you get total electronic wipeout. Imagine a world with no electronics - no Internet, no traction control on cars, no telephones. Us modern people, ever reliant on electricity, would ruin society. The stocks would crash, governments would fall, medical practice couldn't issue electronic-reliant practices. The world as we know it would shatter, all because of a big enough flux in the sun's gravity.
I put my bid on the sun.
