My top 3 would be probably
1. Chainsaw Man - Kick Back. It's the gift that keeps giving; every time that you watch it you'll find something new, and the more that you like it.
Don't get distracted by Power wearing a bikini, or the movie references - look at the details. Makima taking Denji for a walk, Denji seeing carnivorous flowers (a dangerous love), golden balls rolling... Mother's Basement does an amazing job exploring it, way better than I could.
2. Elfen Lied - Lilium. The anime aged poorly, and it's a weak adaptation of the manga, but damn! This OP is beautiful. And the Latin mass actually makes sense in the light of the manga's story:
translation of the lyrics into English
The mouth of the just will practice wisdom
and his tongue will utter judgement.
Blessed the man who endures temptation
Since, as he’s being tested, he’ll take the crown of life.
O Lord, source of virtue
O Lord, divine fire, be merciful
Oh, how holy, how serene, how kind
how lovely the virgin is believed to be.
Oh, how holy, how serene, how kind
how lovely, oh chastity lily
Switch the genders for a moment and everything falls in place: the "man" enduring temptation is Lucy, asking for forgiveness to a "chastity lily" called Kouta. And she's devoted to him almost religiously - because, even if she's a sinner, he's the one accepting her, giving her peace and serenity, pleading in her defence, as if he was her personal Virgin Mary.
3. Sorairo Days - Tengen Toppa Gurren Lagann. The song is great, don't get me wrong, but what makes the OP really special for me is that it isn't just "a single OP". The scenes shown in the opening evolve alongside the story in the anime, while keeping the same catchy song. For example, here's what the first ten seconds of the opening tells you:
- up to ep8 - it's the story of three people: Kamina, Yoko, Simon.
- from ep9 to ep16 - it's the story of a whole team that includes Yoko and Simon but much more people.
- from ep17 to the end - it's the story of a boy growing up, Simon.
Except that it isn't just those ten seconds. It's the whole opening that evolves, throws back at earlier events just to push forward, in a way that feels completely natural and consistent with the theme of the anime series - evolving and going forward.