Honestly, I'm a big fan of the Krime Spree. Internal Recoil Comp of 1 means that you can go to Simple Full Auto attacks with only a Gas Vent 3 (or an underbarrel weight, electronic firing and an extendable stock). It's a little light on the accuracy, but a personalized grip and a SmartLink will take care of that. It's got a fairly decent clip as well, enough for five of those attacks before needing a reload.
If you want to push for a single point more damage and a little more flexibility in firing modes, go for the Ares Sigma-3. The biggest thing you gain there is another point of damage, all of the firing modes and a 50-round drum. I prefer the Krime Spree because of that sweet sweet internal recoil comp, but you can argue that you can approach it because the standard foregrip can be combined with an underbarrel weight.
If we extend into the strictly illegal SMGs, I think you can argue that the HK Urban Combat is your choice for stealthy SMG and the FN P93 Praetor is your choice for going loud.