Picard falls into a mind trap that is actually an extinct race's final attempt to acknowledge its existence.
I rarely rate any episodes in the high "9s", and overall, I think that ST:TNG was a mixed bag of over-heavy messages and thought-provoking episodes. Other examples of good episodes from this series such as "Yesterday's Enterprise" and "All Good Things" are well done but don't stand up to "Inner Light", one of the best one-hour dramas I have ever seen on television in the last 40 years.
The story is well-paced and deceptive, it begins with Picard seemingly abducted and setting up an expectation of "how does he get himself out of this annoying situation?" As he lives an entire "life" of another human being, the characters he interacts with and the situations he finds himself embroiled in are well-drawn and realistic. Even the "planet's" technology is a nice blend of some advances and some traditional and more primitive community rituals. Many remark on how this episode shows another side to Picard, how he could grow in an environment of having a family and a long, satisfying life. What strikes me as even more powerful is the sadness of the fate of a planet and a race, knowing they are going to die and hoping like many individals to just leave a memory of their existence, triumphs, and failures...the expression of this through having another person actually experience it in their mind is very powerful.
The Star Trek franchise often hit upon elements of tragedy and pathos -- in my opinion, the "Inner Light" is its best example of combining science fiction and its implications on the human condition.