I really love oneshots as well. If you don't mind a range of tones and the occasional short story collection, here's a list of some I enjoyed. Hopefully you'll find some stuff that's up your alley.
Mizukami got his start as an artist by being able to write a lot of oneshots on a short notice. 'Walking the Void' is one of my favorites.
Nickelodeon, Melancholia. Dowman Sayman is very good at condensing stories into single eight-page chapters, although there is some interconnectivity between the individual stories now and then.
Tetsuya Toyoda's collections Koohi Jikan, Goggle. Some recurring characters, but largely stand-alone stories.
Gishi Sanjiden and Yuugai Muzai Gangu. Shino Ura wrote these two great collections of stories around the concept of perception, then seems to have disappeared off the face of the Earth.
Most of Kago Shintaro's bizzare gore manga are oneshots or written as sort-of-anthologies. Paranoia Street has some of my favourite situations, but the overall work shares the same characters so it doesn't quite count as a short story collection.