What's your favorite anime opening/OP?

There's no way I can pick just one.

Stein's Gate's OP with the song Hacking to the Gate is probably my favorite overall including both visuals and the song itself.

Prior to watching Stein's Gate my favorite was Shakugan no Shana's first OP with the song Scarlet Color Sky.

I also really liked the OP for Mahou Tsukai ni Taisetsu na Koto (Someday's Dreamer) with the song Kaze no Hana which was my favorite OP before that.

Honorable mention to Honey and Clover for having the most WTF OP out of all the anime I have ever seen.

If you're looking for something else to listen to, basically any OP by Yoko Kanno or performed by Kalafina is good. e.g.:

Hikari no Senritsu from Sora no Woto
Tank! from Cowboy Bebob
to the beginning from Fate/Zero (EDIT: I originally said "both OPs" but I got mixed up with one of the EDs -- the other OP is oath sign performed by LiSA)
the Ghost in the Shell SAC OPs -- particularly Rise fron 2nd Gig

A few others that I enjoyed are:

Asterisk -- Bleach's first OP
Only My Railgun -- A Certain Scientific Railgun
Choir Jail -- Dusk Maiden of Amnesia

but there are way too many to list, so I'm going to stop there before I write a novel.

Edit: Added links. Thanks @Davel23 -- I've been trying to stay off of YouTube lately and didn't know about that site until you posted your comment!

Evangelion - The Cruel Angel's Thesis

Posts question to op as op and doesnt answer. Leaves

That would be "What's your favorite anime, OP?"

But I guess I have to edit the title anyway lol

Nichijou.

Op 2 kicks ass as well
The EDs are also absolute works of art. And that ultra-long pan of the town is a joy to watch.

Probably Serial Experiments Lain. Duvet by Boa.

I usually watch an OP once for a show and then skip it each time afterwards. That being said, there are a handful that I remember making the choice to always watch each episode. In no particular order:

(NSFW) B Chiku Sentai SOX - "SHIMONETA: A Boring World Where the Concept of Dirty Jokes Doesn’t Exist" - I always laughed at the visuals of this more than anything. This show was wild.
Onegai Muscle - "How Heavy Are The Dumbbells You Lift?"
Idol - "Oshi no Ko"
Zeitaku na Saji - "Campfire Cooking in Another World with my Absurd Skill"
GO!!! - "Naruto"

Pani Poni Dash! - Kiiroi Bakansu

Honorable mention - Galaxy Angel - Bang! Bang!

I will never skip the opening (song: Tombi) of Trigun Stampede. It made me a fan of Kvi Baba, who I'd never discovered before.

The tone and lyrics make it the perfect Vash theme song.

Conan during the who is bourbon arc. I was pumped during that arc. I skip 90% of episodes not based on the manga so it only played for like 6 eps or so.

Psycho pass, all. Even the one I initially didn't like got an animation change which somehow saved it.

Preserved roses. Actually almost any tm rev or Abingdon boys school. It might have been an ending but the Tokyo magnitude 8 Kimi no uta. Darker than black, several Gundam and similar, etc.

The opening of beastars has amazong animation and some kinda silly vocal maneuvers that all together I liked more than the whole anime.

One or two from the old Inuyasha that went for like 509 episodes and didn't even finish the main plot.

Several themes from steins gate game and anime. They always bring back the feels.

The entire sound track of natsume yuujinchou. That also gives me nostalgia hits.

Aku no hana. The anime was strange to say the least but that song really layered on the stress.

All the kalafina songs in fate/¿? I think /zero. I really like kajiura's music. Heavenly blue used in aldnoah zero too. The lyrics are really nice and fit the show really well.

Probably tons more. Like one for every 5 bad ones though.

We Are ! (One Piece)

God yes. Both the original and the special version sung by the crew.
- yeah, with this one also https://m.youtube.com/watch?v=pbJ6kEORvwM they are my preferred ones. I don't know animes at all but they simply seem perfect to me :)

Hero's Come Back from Naruto Shippuden

Honorable mentions:

SAO Crossing Fields
Cowboy Bebop Tank
Gurren Lagann Sorairo Days

Harukaze by Scandal

I thought my answer would be Ghost in the Shell Stand Alone Complex, because the theme song is so good. However I just rewatched it, and it's full-on early 2000's bad 3DCG.

My actual favorite overall has to be One Punch Man then.

Ok I got more

Komi can’t communicate

https://m.youtube.com/watch?v=xHlx3rx5Bew

Gundam iron blooded orphans

https://m.youtube.com/watch?v=82HM0z6qd20

Black lagoon

https://m.youtube.com/watch?v=dWfxcRj39tg

https://m.youtube.com/watch?v=vIhQ9zya2-g

Gundam the witch from mercury

Any jojo op but especially great days

https://m.youtube.com/watch?v=mU3vgXUKeFM

And this one

https://m.youtube.com/watch?v=BaoWJnC3V_o

My favorite opening is probably Akame Ga Kill. Either that, or Angel Beats

Inner Universe from GitS: SAC - if cyberpunk had a theme song, it would be this.
No Separation from Heaven Official's Blessing - a warm, slowly building love song with a hint of loss and longing.
A Cruel Angel's Thesis from NGE - an energetic, action-y song with darker undertones.

https://animethemes.moe/anime/ghost_in_the_shell_stand_alone_complex

Gives me chills, I love it.

PUT YOUR GRASSES ON

Welcome to the N.H.K. ("Puzzle" by ROUND TABLE feat. Nino) is one of my favourite openings

I love that OP and the ED Akachan Ningen is great as well.

The one that's always stuck with me is Blood+ third opening to the song of Colors of the Heart by UVERworld.

ClariS - Irony - OreImo season 1 OP

It's so gahdamn catchy

Another I remembered and wished I'd mentioned - Azumanga Daioh

It's so obviously a nod to Sparks, which is fitting for such a surreally goofy anime. And that's a neat bit of recursion in itself, since Russell Mael at least (and possibly Ron too) is an otaku. I've never seen anything official about their influence on the song, much less about their reaction to it, but I like to think they were flattered.

I'd have to pick Butter-fly from Digimon Adventure.

It may not be my favourite but the english dubs of the early one piece openings and endings make me tear up with nostalgia. I haven't even followed the anime since many years

I have a huge nostalgia shaped soft spot for the early One Piece openings.

I really like "Lay Your Hands on Me", the opening for Kiznaiver, despite the anime itself being a huge disappointment.

Death Parade's "Flyers" is a masterpiece.

Others that come to Mind:

ACCA 13
BNA
Flip Flappers
Made in Abyss
OPM
Steins Gate
Trigun Stampede

I legit can't choose. There's some really good stuff out there. In terms of what I've listened to the most, that's likely Shout and Gravity Wall, the OPs of Re:Creators. The story is kinda meh, but man does that show has some high highs, and the OPs are one of those.