Here's a few and I'll give you the situation
ASM1
Stock, Foregrip, Red dot (I customed this to your play style, I typically run quickdraw or extended mags for better kill feeds)
I also have one with Stock / Quickdraw / Advanced Rifling
Lightweight, Gungho with Toughness and Scavenger.
I typically use this class for more MR maps. The ASM1 is by far one of my favorite SMG's in the game for this. If you can pop shot it from MR / LR you can still get the kills.
KF-5
Same setup as above. The only difference is this is your CQC maps only. The KF5 is useless if you don't plan on being right up in the Enemies face. If you're anything like me that's your sometimes preferred playstyle and up close there's not a gun that melts better aside from maybe the MP11 Goliath.
Bal27
Attachments: Foregrip, Extended Mags, Stock
Kill streaks remain the same
This is probably by far my favorite class. Even with the nerf to the Bal it is still one of my favorite guns in the game. It's diverse in the fact that it allows you to switch up your play style with out switching out your load out.
Now, I also have an ASM1 / Bal-27 class that assists with going for DNA bombs. I typically run low profile and cold blood on that class with the same attachments and the Killstreaks being a hyped UAV with threat detection red arrow and a System Hack with Extra Time, Disable Exo Suits and Flash on it.
This is for more defensive play obviously but allows you to play more aggressive once you get your UAV unlocked. You can chose to put extra assists on your UAV to carry you to your System hack if you'd like I typically do as once the UAV goes up people get slayed on the opposing team.