I agree, MP5 kinda sucks. UMP is waaay better, thing drops you in 4 hits without stopping power, that's less that most guns.
Best lightweight class: Silenced UMP, marathon, lightweight, and whatever, usually commando, though ninja pro might be awesome too. either a M93 Raffica or Akimbo 1887s to be a bitch. lol. I agree, theyre cheap, they should/are getting nerfed, but it's really fun when you get to use them.
The ACR isnt as good as most people think. It's pretty weak, and the iron sights are kinda inaccurate. Here's a chart I've figured out.
Stopping power------>>> Accuracy/Fast shooting
SCAR ---- TAR ---- ACR
I'm not sure if I like the ACR or TAR better, either way, the M16 rocks.
BTW: unless you're going for a nuke, I'd recommend putting your last killstreak reward as a Pavelow. Kills nearly as much as a chopper gunner, you can still run around and kill people, and you can get it in 9, instead of 11 kills.