Last night I was just using the default assault load-out and it worked really well for me. It's just a basic M4A1 with sleight of hand, stopping power, steady aim, and then I've just been equipping it with whatever attachment I needed to complete challenges.
It's funny, I never really got into sleight of hand on the first game, but now that I've got the pro version unlocked I really love it. Being able to bring your gun up quickly out of a sprint is NICE.
As for the M16, I just can't get into it, both on MW2 and the first one. I just don't like being restricted to bursts. When you're on target it's great, but if you miss and your enemy is using something fully automatic then chances are good that it's game over for you. I just keep going back to the M4A1. It's got decent power, decent mag size, it's pretty good at range, and it's fully automatic, so I can either snipe fairly effectively or I can rush and spray with it. The TAR is somewhat similar, but I think it's got a bit more recoil.
Haven't unlocked the ACR yet. I'm still only a level 40 non-prestige, lol...