Oneshots aren't as popular as manga in the manga reading communities on the internet.
Salam (peace)
This posts where I talk about some interesting manga One-Shots I found. Because these manga are short and can be read in less than 10 minutes. They have different charm than manga series or the short fiction.
There are more One-Shots than series in Japan but those are rarely translated. So finding ones in English is very hard. Most of the translated ones are the pilot chapters of the popular manga (these are mostly published before the manga series to give the publisher an idea how popular the series would be). and other manga one-shots that are compiled with a series volume of the same authors.
So In this list, All these one-shots are interesting ones I came across.
I'll also include two and three-shots as I consider them One-shots too.
Naruto Pilot

by: Masashi Kishimoto
It's pretty famous series and probably a lot of readers know about the pilot chapter... but I listed it because it's very different than the series. It follows a fox (in a human body) called Naruto who has a very bad personality... Not getting along with anyone in his village. he is sent to the city and told he must make a friend there.
I actually liked it more than Naruto's first chapter. (But that is awesome too)
Death Note Pilot

by: Tsugumi Ohba and Takeshi Obata
Like Naruto above this is a very popular series and a one-shot... I put it here because it's also different than its series. And takes the opposite direction. It follows the middle school boy Taro Kagami who finds a (magical) book and uses it as dairy.. he writes how he got bullied in school and in the next day he finds those classmates dead.
Aru Asa Okitara
(One Morning I Woke Up)

by: NISIOISEN and Kawada Yuuya
A story about a girl who wakes up with no memories. Then he remembers he is actually a boy.. not knowing what happened or what to do he decides to go to the school of this girl... and he slowly starts remembering. The ending is kind of bittersweet but it's really good!!
Boku no Hero
(My Hero)

by: Kohei Horikoshi
The oneshot of "My Hero Academia". it's published with the final volume of the Kohei's previous manga: Oumagadoki Zoo.
The oneshot is about a salaryman in a world of heroes and villains.. He joined the "hero" business so he can get hold of hero items legally... but he uses them without a licence and puts himself in a trouble. It's as awesome as the main series: My Hero Academia.
It seems that I listed the oneshots of popular series only here... but if I did post about this again I'll probably post more obscure ones. I wish there was a group that translates amateur manga from contests like Treasure. But well, they're not popular enough to be translated even though many of the winners get to be successful mangaka in the future.
I hope you have enjoyed this post. Tell me if you want more of it :)