made a compilation of what I think are the best openings in anime
Outlaw Star - Into the Night
Kids on the Slope - Melody of the Slope
Nana - Lucy
Serial Experiments Lain - Duvet
Bleach - Alones
Samurai Champloo - Battlecry
Spice and Wolf - Tabi No Tochuu
Cowboy Bebop - Tank!
Hajime no Ippo - Under Star
Haikyuu - Ah Yeah
Jojo's Bizarre Adventure - Bloody Stream
Nodame Cantabile - Manazashi Daydream
White Album 2 - Todokanai Koi
Konosuba S2 - Tomorrow
Yosuga No Sora - Hiyoku no Hane
Spike and Wolf S2 - Mitsu no Yoake
Just Because - Over and Over
Yu Yu Hakusho - Smile Bomb
One Piece - Brand New World
Bleach - D-tecnolife
Sangatsu No Lion - Sayonara Bystander
Rurouni Kenshin - 1/2
Rakugo Showa Genroku Shinjuu - Usurahi Shinjuu
Welcome to the NHK - Puzzle
Great Teacher Onizuka - Hitori no Yoru
Hajime No Ippo Rising - Yakan Hikou
One Punch Man - The Hero
Kare Kano - Tenshi no Yubikiri
Great Teacher Onizuka - Drivers High
Revolutionary Girl Utena - Rinbu Revolution
Space Dandy - Viva Namida
08th MS Team - Arashi no Naka de Kagayaite
Neon Genesis Evangelion - Cruel Angel Thesis
Hanasaku Iroha - Hana no Iro
Hanbun no Tsuki - Aoi Koufuku
Fullmetal Alchemist - Rewrite
Blue Gender - Break free
Fullmetal Alchemist Brotherhood - Rain
Inuyasha - Owarenai Yume (Never-ending Dream)
GITS:SAC - Inner Universe
GITS:SAC 2nd GIG - Rise
The Twelve Kingdoms - Jūni Genmukyoku
Escaflowne - No Need For Promises
Overman King Gainer - King Gainer Over! (OP written by Yoshiyuki Tomino, how could it not be great)
Gundam Wing - Just Communication
Dirty Pair - Russian Roulette
Mazinger Z - Mazinger Z
Lupin III (pretty much every iteration is dope)
I know I'm forgetting a lot...
and while not an OP theme, Yuki Kajiura's Canta Per Me (from Noir) is (sonically) one of the greatest songs to grace any anime period.