You can view the list, and their reasons why, here.
It includes episodes with Klingons, Borgs, TOS clad Enterprise members, the return of Brent Spiner as Arik Soong (Data's creator Noonien Soong's grandfather) and Augments, Mirror Universes, Orion slave girls, Riker and Troi and more!!
Some people may actually scream at the choice of ''These are the Voyages'' (s4,ep22) as being in the top 10 but I myself really liked the episode (besides the fact that we learn that fan favourite Charles 'Trip' Tucker was going to bite the bullet on a mission in a few years, before the creation of the Federation when this episode takes place, on the Holodeck 100s years in the future - confusing no? Haha). I enjoyed seeing Riker and Troi again and the use of the holodeck to convey the story.
Read it all and chime in if you like.