Enough talk of the M16... that is the best gun all around, which is why I hate it. Anyone can use it. It has the best accuracy. The most powerful. So here's my advice to you about the M16... when you're thinking about using it, DON'T! Just... please... for the sake of my sanity, stop using that blasted thing. It's the easiest gun to kill with. EASIEST. Question me? I've gone over 100 kills in Headquarters, not in Shipment, with only like 20-30 deaths. I have not been able to do the same with another gun yet, but I'm getting close with a sniper.
If you want to use a silencer, good choice by bringing up the SMGs. P90 does shoot more rapidly, and if you are going for up close, that's what you should use. If you're going to use some range, then the MP5.
To the rest of the guns that were mentioned, the G36 is great with a silencer, as is the AK-47. I love when I have a sniper and I suddenly find a silenced AK-47 on the ground. That makes me more giddy then I am at Christmas. Another one of my favorites is the AK-74. And that's an SMG, so I think it'd be probably the best one to use out of all of them.