Queen of Hearts. Not just out of revenge, but because the message delivered with Art's death sounds like it could have been her handiwork, and because she's always mysteriously online whenever the mafia makes a decision about who they'll kill.
Mlle's exactly right. I not only ordered the hit, but I actually ordered one that I think was a little worse (I wanted Art to choke on it). JJF cleaned it up a bit, but it's still mostly my text. I knew I had to wait for the right moment to kill Art, and he presented it when he started talking smack.