Yay! A long awaited topic, i've been waiting for this to come up. Prepare for my rambling now :getdown:
Well, my preference of spellcard is "fun", whether it's fun to watch, or fun to dodge, or any other "fun". I always love spellcard that have a lot more than it appears.
Guess I'll make it short in the end, don't want people to see the great wall of text.
First off :
Most of Youmu's spellcards (and non-spell), especially ones that fucked up with your sense of direction, with the added tension. Her spellcard is actually simple when you know what you're doing and thinking ahead of things, but when you rely on your reflex and intuition, dead is on the way. This battle is really a mind trick that are implicitly created by ZUN (I wonder if he's really drunk or not when he creates the spellcards.)
Divine Spirit -Fantasy Seal- ~Blink~ : This spellcard really is fun to dodge, at first you just gotta deal with those homing amulet "easy, easy~" and then she dashes straight to you and shot you with a stream of fast danmaku that makes you go "shit." and boom, you either died, or have a lucky escape, then died the next wave (or at rare occasions, managed to capture it.)
Heaven Sign "Scorched Earth Mandala" (Shou's spellcard at DS scene 7-5, don't know if it counts or not, but, whatever) : This. You really don't want to be in the same cage as Shou. This spell is really fun, that feeling when you eventually figured out how to not get eaten by the tiger. Heavenly. I actually spend 2 whole damn hours to figure this out (yeah, i'm an idiot).
In term of epicness, I'll go with
Descent of Izuna Gongen (you already know why) : my reaction is mostly the same as EthanSilver there. When I first faced this spellcard, I already cleared some of other game extra's and this is my actual "mind" quote: "Alright, last spellcard i can do this" First 5 seconds *already very tense* "Okay, there's no way it'll gonna this easy, there will be more than this, prepare for the worst" yellow danmaku appears *panic mode on*, Blue ones appears *Died the last live and bump head to the table* :V
In term of most beatiful,
Saigyouji Flawless Nirvana comes to mind : It's just dazzling and it feels like you really look at a sakura tree at full blossoms, a real nirvana.
Dohoho~
And here I am dodging bullets like my live depends on it. But wow they really know how to find a loophole, just like those stage part of IN extra where you can just stay at the bottom left corner without getting hit at all.. :fail: