What do I look for in a sympathy card? Two elements: design/art and printed verse/message should convey your personal style but more importantly: it should feel compatible with the grieving person and circumstances of the loss. You'll know it's the right one when you find it.
Avoid splashy, crazy, chaotic designs, and plee-e-e-e-ze: no musical or recordable message cards!!
But wait! Don't just write "I'm sorry" and sign it! What's memorable about that? Set the card aside and, on a plain sheet of paper, start composing a message. Use the name of the deceased, make note of the relationship--and qualities you admired in the deceased or the bereaved.