If you want a truly comprehensive experience, then watch every episode.
Agreed. To get the full experience watch every episode of every variation of Trek. That's the only way you are going to determine what is 'essential' to you.
It has become clear to me that this really is the only way to have an 'essential' or 'comprehensive' experience, unfortunately I really don't have the time to dedicate to that kind of viewing experience. However, I know enough about the setting and the characters that I feel confident changing the focus of this project to be more focused on certain aspects of the show.
This sort of approach will still be highly subjective but might help others experience the parts of Star trek they loved, such as the Borg invasion of the Alpha Quadrant for me, or perhaps never explored, such as the Dominion War for me.
Plus, this focused approach is really pretty fun.