Some of my personal favorite openings:

• Elfen Lied: the anime itself was kind of ok but the opening and first like 10 minutes are so memorable. Been decades since I've seen it so maybe the plot is better than what I remember. Doesn't change the fact that this opening is hauntingly beautiful
• One Punch Man: such a good hype up song. The song makes you feel like you can do anything
• Pokémon (Kanto League): another great hype up song though I think a large part of it is how nostalgic it makes me since it was my first anime

And Endings:

• Inuyasha (Every Heart by BoA): like Pokémon, I think a good reason I like this song so much is the nostalgia. But even taking that out, the ending is a great example of jpop from the early 2000s. Fun fact, the singer is actually a kpop star and has also released this song in English and Korean. Growing up only hearing the Japanese version, it's super weird to hear it in any other language especially from the same voice.
• Jujutsu Kaisen Season 1: the chill hip hop vibes and cool animation are such a fun way to palate cleanse all the gore and seriousness of each episode
• Asobi Asobase: everything about this ending is great. The psychotic visuals, the cute anime girl vocals, the heavy metal backdrop. What makes it even better is just how much it contrasts with the show's opening. Many of the comments for both videos point out this hilarious disparity.