The surface of Earth has been rendered unlivable. Due to years upon years of pollution, humanity has been forced to build cities to rise above the planet's filthy surface and adapt to this new way of life. Not only this, but there is also a thriving kingdom of mutants lurking below the billowing clouds of smog and pollution.

What is your favorite anime closing theme?

Most (all?) of these will probably be drama EDs.
Not least because I believe a good ending should always convey a feeling of completion, calmness and a dash of sadness.
Without being boring or even depressing in the process, that is. So, let’s see what we have here (song names are YouTube links)… Omoide Good Night from the utterly unknown W Wish.
Lowest in the list, but it managed to survive way, way longer in my playlists than it ought to have.
Simple, but instantly recognizable. Itsuka Tokeru Namida from the almost-as-obscure Uta Kata is high on this list as well.
Despite being the potentially most sad and haunting anime piece I ever came across.
That voice… priceless. Alicemagic from Little Busters is an awesome candidate.
So much verve and energy.
Acknowledging pain, but screaming at you to never give up, no matter how harsh the path. Asu he no Namida from Onegai Twins.
Driving, wistful, and soothing all at once.
Good melody, interesting vocals.
Actually, I had a really hard time deciding between this and the last one, buuut… … but in the end, my cake goes to Ashita e no Kaerimichi from Natsuiro Kiseki. Mind you, I could never really get into that whole gimmick of “seiyuu forming a idol group parallel to the one in the series” that the show had going.
In fact, I never found all that much to enjoy in SPHERE’s other productions. Even so….
at least the OP and ED of this series are easily among the finest JPop tracks I have ever heard.
Starts small, but slowly builds into a truly awesome melody and chorus.