Biggest miss for a lot of people in my opinion was Gate. That was my crack this year.
I did like the premise of Overlord, though. They ended it at just about the right place as well because the light novels after that start to require a pallet cleanser between them.
Gate was freakin' spectacular...wait, why am I saying "was"? Season 2 just started a week.5 ago and it is still spectacular!
Also, there was the finishing up of Aldnoah.Zero in January 2015 with the 2nd season wrap up. While it wasn't as spectacular as the 1st season, it was far better than many of the Gundam shows.
World Trigger is a good long-standing series starting from late 2014 (Oct) and is still going strong at 60+ episodes. A very detailed, in-depth anime (if a bit 'cheesy' at times).
Oh, how about the Heroic Legend of Arslan? That one is a very enjoyable medieval times war series.
Noragami picked up it's 2nd season in 2015, 5 years after the 1st season aired! Still in good taste and still held up greatly with a very enjoyable plot line.
K (King) also picked up its 2nd season in 2015, after a 3 year hiatus from the first season's 2012 release.
Seraph of the End is an okay post-apocalyptic vampires vs. super humans show with a few added twists.
Oh, and one big one: Ushio and Tora. Great characters, enjoyable story. Funny sometimes, but not an overtly pushy comedy compared to trying to keep it more down to earth.
Overall 2015 was a great year for anime...and not just "Gundom, man!".