When I started reading m/m books, I was delightfully surprised to find so many great series, and I don't mean great for just that genre, but fantastic overall. So here's some series I've really enjoyed, although this is certainly not all of them (and best of all, some of these authors have more than one great series):
By Josh Lanyon (the Adrien English Mysteries):
Excellent mysteries, great writing and at one point in the series, I have to say I have never hated a ficitional character as much as I did Jake Riordan. I could've thrown my Kindle.
By Abigail Roux and Madeleine Urban (the Cut & Run series, with more to come):
Loved both main characters, even when they sometimes hate each other.
By Jordan Castillo Price (the Psycop series, with, hopefully, more to come):
I love the main character Vic and his warped sense of humor. I also love paranormal, so this was a great series for me.
By Mary Calmes (the Warder series):
Paranormal, with each book focusing on different, but related characters. Novellas. Sinnerman was my favorite.
By Lynn Flewelling (the Nightrunner series - not strictly m/m, but mainstream fantasy)
Excellent fantasy world with great characters. There's also a complimentary series set in the same world, but in a different time period and also excellent.
Can't wait to find my next favorite series. Feel free to email me your suggestions at finnmarlowe@yahoo.ca! You can check on Goodreads to see if I've already found it first, if you like.
As for myself, I'm hoping to someday finish A Trick of the Light (next after A Thread of Deepest Black) and to edit down to manageable size A Kiss of Tears and Thorns and maybe have a series of my own...