Some people don't like series much because they hate having to wait for the next installment. More so with this type of series, because with the community style you can read them out of order and still usually "get it", and since each couple get their HEA in their book, there are no "cliff hangers". I know some readers like to save up and then read them all at once as another option. I don't mind a cliff hanger as a rule, even though I'm quite liable to whine about it here, it doesn't put me off buying the books as they arrive.
Now picking my favourite series is a bit like picking my favourite child. Luckily I only have one of those, not so with series. So if I missed anyone I apologise and I hope others chiming in with their favourites will mention those I don't. In no particular order:
First up, the Cambridge Fellows mystery books by Charlie Cochrane. I confess I have not read all eight of these yet. It's certainly not because I don't think they are worth a read, they definitely are, but I just haven't got there yet. So many books, so little time. But the adventures of academics Jonty and Orlando are great fun to follow as the amateur sleuths solve a number of crimes while dealing with their relationship in the early 20th century.
More mysteries come from Victor J. Banis with his Deadly series of books. Who can resist flamingly out cop Stanley and his macho straight(?) partner Tom as they solve crimes. Of course it gets really interesting once Stanley and Tom become a couple and head off on their own as private investigators. Another series I have not finished but it's on the list. Two to go.
I defy anyone not to fall in love with Nicky and Brandon in James Buchanan's series. In-the-closet cop Brandon and goth-boy Nicky are a perfect couple whose D/s games make for some smoking hot reading, along with some good crime solving. Even though I wanted to smack Brandon more than once for the way he treated Nicky, I was all smooshy and "awwwww" at the end. So it's good. :-)