I don't like series.
I suppose I could deal with mystery novels that can be read as standalone, but the thing is I used to be on a tight budget until recently and did not want to spend money on something that is less likely to have a reread value.
But while such examples might technically count, I believe that's not what's on one's mind when one says "series".
The string of novels in a single continuity. Following the same characters more often than not. Or, at least, chronicle a sequence of events. Where you have to read the next book to get closure, and have to have had read the previous ones to get what happened. And let's not forget the cliffhangers.
And I will never ever know how anyone can be so arrogant to presume they will be able to produce several books of decent quality on the same subject, couple of years apart. Even many series of standalones lose steam along the way.
I won't count trilogies/quartets/quintets here.
Naturally, there are a few exceptions.
I'm reluctant to call Doyle's Sherlock Holmes opus "series". But it is one technically.
In Terry Pratchett's Discworld, books are standalone, but there are a few where knowing previous events helps (but is not necessary, in my opinion), forming several sequences under one large hat. It's one of the few that's solid overall, even though it has its ups and downs - but it's not a steady decline.
The only "series" in the style that is popular nowadays that I like so far is Tanya Huff's The Blood Books.