Okay, really tough toss-up for me between Great Fairy Wars and Impossible Spell Card.
First, Cirno's always been one of my favorites, and that being the second to feature her as playable in a shooter, and the first to solely star her, was a big plus, and between freezing bullets and the hilarious dialog, it earns major points. Out of the four, I've played this one the most.
But on the flip side, I've also liked Seija as long as she's been around, and she's my favorite boss fight ever, so having a game featuring her was fantastic. I loved the concept too, where everyone stops following the rule about making spell cards escapable and she has to cheat her way out, including and especially by using other characters' stolen items. Though I haven't played this one as much, It barely wins in the music department, and it's got a bit of a darker tone that I like just a little bit more. And to boot, the levels are a bit easier to digest since you don't have to play through the whole game in one sitting to beat it.
So it's pretty darn close, but I'd say I'd lean on the side of Impossible Spell Card for this, even though they're both flippin' cool.