I just started rewatching it for the fourth time(rewatch value right there) and noticed I hadn't written a review.
For me Stranger holds special meaning since it's the first ever kdrama for me(not taking movies into account) and it was such a high quality one from every aspect(story, cast, overall production) that immediately set the highest standard for me. After that it was difficult to find equally good productions, though I have found some.

The story has good twists and keeps you guessing and even when it's not, it still has a good execution that it's still as interesting. The cast is amazing. Solid performances from everyone. It starts off a bit slow but after the 6th episode it's addictive. And as far as I remember, there weren't any of the classic mechanics in writing that make it seem contrived. The characters and their actions have consistency.

I really don't want to say more and rather have you watch it. Even if it's not your favorite I think you'll still find it a very good one.