My favorite is the Chicom with Silencer and Fast Mags. It's one hell of a weapon and it drops people really fast. It's also my easiest SMG to get headshots.
Working through SMG gold first so my exposure to anything else is limited and have tried the Chicom yet, but I have developed a real appreciation for the Vektor(sp?). My least favorite is the msmc. Currently trying to work through the two wildcard challenges where you are required to have two attachments on the secondary and get one kill with each and I do like having the option where I have one silenced primary for going spider monkey and one setup that is loud and for a more defensive posture. Once I finish up the two challenges and then max out the FHG I will probably return to a double primary setup for most classes.