I have a few oneshots, and they aren't bad, but I love to get emotionally invested in characters, grow to love them over time. You just can't do that in a oneshot, so I prefer series.

________________

Short series. Long series often start to drag, besides, I like complete stories, and long series are usually either old or ongoing, though there are some pleasant exceptions. So: 2-10 volumes.
I still read oneshots, though, I like being able to finish a manga in one sitting when I don't have much time. But I don't go specifically looking for them like I do with series. If it's a recommendation or my favorite scanlator releases some, and I like the description, I read them.

I like both.

One shots are nice for quick reads if I'm bored and series give me a whole world to dive in.

All in all I like series better tho, most one shots are a bit too short to actually care about the characters in them.