Three Days of Happiness 
 
this manga is by no means hidden, i think a fair amount of people know about it. although it's genuinely one of the best manga's i've read to date, take that as you will. 
 
if you're looking for a bittersweet story dealing with romance, loneliness, and a search for happiness this has it all. the read can be a bit jarring at times, but it's ultimately a really sweet pay off in the end. 
 
this manga is short, obviously. you could definitely finish it in an hour or two if you're reading it all in one sitting, but the pacing in it is perfect. everything comes together really nicely and the story's world building, and also somewhat supernatural aspects to it. while the read is short, i can guarantee what you gain from this manga sticks with you forever.