Well to me, my favourite OP/ED's are the ones that make me stay there and listen rather than skip ahead or just leave straight up. Some anime, I've tolerated just because of their OP/ED's. I probably could have put in some VN OP's in here but I skip them 100% of the time because I don't want to get spoiled on the routes in the game.
OP's:
Very good OP, I can tolerate it's BS purely because of this OP. Never skipped this during the first half.
I've heard people say they think this OP is cringey because of their English but I think it's really dope. Never skipped this one.
Yukine is such a brat, but I can tolerate him partly because of this OP.
Yeah this is a classic, no way I was missing this.
Love me some screaming.
Except this is how you actually scream tho, love this OP. Never skipped this one.
If you ever plan on watching Aldnoah Zero, don't watch this because it has some spoilers.
ED's:
Btw Emilia is best girl. Never skipped this one.
Overlords ED's are so sick, So-bin's art is so amazing and it always has the sickest tracks. Never skipped this one.
ED3 is even better than ED2 imo, the lyrics for this song is so fitting for the series as a whole. Ains isn't playing around anymore. Never skipped this one.
This ED doesn't play all the time, but when it does I never skip it.
Another ED that doesn't play all the time, just another one of those amazing songs from Kalafina. Never skip this when it plays.
Kara no Kyoukai 1 ED, many people got into Kara no Kyoukai just because of this song. Berry good ED.
If you haven't or are planning to watch No Game No Life, there are spoilers in this.
There's tonnes more I could have put in here and these are just the ones that come to mind off the top of my head, but I cbf to go through my MAL to look through everything so I think I'll leave it at this before this gets any longer.