As the years have gone on, I've found The Defector to be in my top 3 episodes of the series, yet it doesn't get the recognition from the Trek fan community. I think it served as everything TNG was about, working for a peaceful solution rather than going in guns blazing, and it was probably the best use of the Romulans in the series, maybe second to Face of the Enemy. Also, it was what I wish Nemesis was based on. Heck, if Star Trek: Picard addresses Admiral Jerok's letter home to his family, I will be greatly impressed.
In terms of other underrated episodes from each season:
Season 1: Lonely Among Us
Season 2: The Royale
Season 3: Transfigurations
Season 4: First Contact
Season 5: Ensign Ro
Season 6: Suspisions (I know this episode isn't well liked, but I thought it helped make the Ferengi more 3 deminsional. Before Nog wanted to join Starfleet, you had another Ferengi who believed more in science than profit and he too wasn't taken seriously at first)
Season 7: Thine Own Self
As for overrated episodes, over the years I've come to find The Inner Light to be overrated. I just can no longer get over the fact that Picard was mind raped.