Tie between Mystia and Cirno.
Cirno for her elegant attacks, viz. her first noncard, Icicle Fall normal, and Perfect Freeze are some of my favorites in the series.
Mystia for her surprise factor (Much more so than Kogasa...)- the first time I played Imperishable Night, the darkness absolutely killed me and it was so very much fun! Plus, you gots to love her dialogue~