This is the last day of 2017, and man, what a year it has been.
To me, the most monumental discovery and event of the year is the serendipitous observation of the merging of two neutron stars. This celestial event was observed by both conventional astronomical observatories via the detection of EM radiation (light), and by VIRGO/LIGO, which detected the gravitational waves. To many people, this marks the distinct beginning of gravitational astronomy.
There are already papers pouring out of this event, and many more to come. There are already strict constraints on alternative gravitational theories just from this one event. I expect many more to fall as we continue to shake the tree.
Who knows if such an event will occur again some time soon (or within my lifetime), but this is exciting stuff where a new channel and method to observe such event has opened up. I definitely consider this as one of the top monumental discoveries in my lifetime.
Happy New Year, everyone!