Cowboy bebop is the best without a doubt.
After that I can do a lot of honorable mentions.
-First opening of Kenshin is lovely melodically. The chorus part always gives me goosbumps. Nostalgia binging the entire series in VHS that a friend recorded from canal +
And now i start with nostalgic anime culture from Spain:
-The spanish op of Sailor Moon is also fucking fantastic (its basically a dub from the japanese one) and really nostalgic for me catching it having breakfast before school watching Antena 3.
https://m.youtube.com/watch?v=wbbaisQFKKw
-Nostalgic fun is Chicho Terremoto's spanish op (Dash Kappei in Japan). The whole spanish dub was fucking crazy and we all loved it here.
https://m.youtube.com/watch?v=cN41J1yxSu4
-Los Caballeros del Zodiaco (Saint Seiya) spanish intro. This one a complete and strange, if funny, change. Completely different from the Pegasus Fantasy (with footage from the movie), it sounds like an heroic anthem instead of rock and the most strange thing is that it was made by French for the spanish television, so the guy that sings is clearly french singing in spanish, with really famous funny lines, like "La canciĂłn de los herĂłes" (the song of heroes) sounding like "La canciĂłn de los errores" (the song of errors). Even then, close to spaniards hearts.
-A bunch more like Oliver & Benji (Captain Tsubasa) and Ranma in spanish, Dragon Ball, Doraemon, Dr slump and Musculman (valencian and catalonian), etc...
-The Spanish version of Butterfly from Digimon. Is the same melody of Butterfly, but the lyrics tell what digimons are and the kids trying to get rid of the black gears. Probably the last good spanish dub intro done. (We dont really dub songs anymore apart from the animes for kids).
https://m.youtube.com/watch?v=H7_2Wq9ddCE
But I want to add specially the two famous spanish/japanese coproductions during the 80's
D'artacan y los Tres Mosqueperros (Dogtanian and the 3 Moskhounds)
La Vuelta al Mundo de Willy Fog (Around the World with Willy Fog)
Im really proud of this two, that there was once a day we could work with japanese anime studios to create fantastic series.
The Italian/japanese coproduction Sherlock Hound (spanish op here that was a dub of the oroginal italian one) was also incredible, Miyazaki worked on this series, and it shows.
https://m.youtube.com/watch?v=W1ksMXKC6IE
That siren sound at the beginning triggers my nostalgic weekend 8am mornings, when the block of good stuff starts airing.