Please note, I have a lot of ties for this question because I fucking love the attacks in these games but
1. Selaginella 9 (EOSD Stage 3 Hard/Lunatic Midboss Card 1) especially, but most of Hong Meiling's (EOSD Stage 3 Boss) spells. They're just so lovely~

They're colorful and appealing to the eyes.
2. Pretty much every Final Attack in the series (And some Extra Finals), but from what I've seen, here are my favorites, the better ones at the top of the list.
- Danmaku Bounded Field
- Mountain of Faith
- <Insert 10(0 < x < 10) here> Percent Reflowering
- Red Magic/Scarlet Gensokyo
- Hourai Elixir
- Border of Life and Death
- Flying Fantastica
- Mishaguji-Sama
- Hourai Doll
- Hell's Artificial Sun
- Newborn Divine Spirit
3. Suwako's First card, Two Bows, Two Claps, One Bow. Honestly, this is the only card where it looks awesome to have everything and the kitchen sink thrown at you.
I'll say more if I feel like it.