

“I have been and still am a seeker, but I have ceased to question stars and books; I have begun to listen to the teaching my blood whispers to me.” ― Hermann Hesse
Favorite Anime OP?
Original Answer: “Resonance” by T.M. Revolution (Soul Eater)
New Answer: “Nirvana” by MUCC (Inu X Boku SS)

As I mentioned the first time that I answered this question, I found the question a bit vague considering I wasn’t entirely certain whether the creator of the challenge was asking about favorite OP as in just the song or favorite OP as all inclusive of the song and the artwork accompanying the song.
I’ve been doing a bit of reflecting on my answer and despite the fact that I found myself to be rather immature about how I went about commenting on the question, I still find my choice to answer based upon music, timing, and animation to be the most apropos.
I also realized something else as I was re-reading my first time posting on this particular question. While the 5 songs that I mentioned previously are still among my favorite openings, I listen to far more Japanese music now than I did even a year ago and as such, my tastes and appreciation for OPs have changed, which I believe to be a good thing.
While “Resonance” is one of my favorite songs still, T.M. Revolution/Abingdon Boys School are one of the bands that I follow, and that first OP of Soul Eater is just absolutely phenomenally animated and timed, it is no longer my favorite.
“Nirvana” by MUCC (my favorite J-Rock band) is possibly my favorite song in Japanese—possibly one of my favorite songs regardless of language and genre. I find the lyrics to be meaningful and find the melody to be strong yet hauntingly melancholy, almost like a cry. These things are brought out in the animation of the OP. You have it begin with Miketsukami sending out the paper airplane, which we can follow throughout the OP as we get to Ririchiyo and her desperation to search and find something, ultimately clasping onto the paper airplane, being reborn as if she were Thumbelina in the flower, and having her hand firmly clasped by Miketsukami—the paper airplane he sent out quite literally finding its target in her hand as she finds the thing she was searching for in his hand. I just find it rather beautiful and poetic, especially when I consider the vocalists desperation in his singing. To me, it is astoundingly gorgeous.
Of course, what OP would be complete without the main character running or a perusal of the entire cast, but it makes sense to do so, especially with the unification that the paper airplane’s flight brings.
Since I LOVE music and watch a plethora of anime, there are some honorable mentions from anime that have aired quite recently:

digi-sadist ……. im ok with this
Chibi alien? Uggghhh….
Magical Bride…
No. I refuse to be a freaking Magical Bride. Hell to the no.