Actually, I like trilogies probably best, as a reader. It can be difficult to commit to a neverending series if you didn’t get in on the ground floor, as it were. And if you really enjoy the characters and the world, is a single book ever enough?
Though I’ve seen a lot of reviewers particularly mention how nice it is to see a standalone novel. So I may not be in the majority in preferring trilogies. Also, I’m patient enough to simply wait till the third book is out before reading a trilogy, so then the trilogy acts for me like a single standalone. I’m not sure how many readers wait like that.
What I hate most, though, is when the publisher hides the fact that the first book is a non-standalone fragment of a larger work. I HATE that. If we are going to have a cliffhanger, can we have a clear warning about that right up front?
As a writer, the situation is more complex. In discussing the trilogy phenomenon, this post by Justin Landon at tor.com comments on why that is (among other things): on top of questions about what you want to write and about what your contract(s) say you must write, there are financial considerations about sales predictions and so forth.
Justin thinks the appeal of trilogies has to do more with deep wiring in human psychology, though: pattern recognition, right, and a tendency to see things in threes.
Or else he suspects the trilogy form isn’t really as dominant as it seems, with the appearance of dominance created by confirmation bias. Leading to a really clever last line. Yes, Justin, I think we all have that particular problem with confirmation bias.
Anyway, if you have a minute, click through and read the whole thing. Do you have a strong preference for standalones, trilogies, neverending series?