Only bolt action snipers require some skill, but quickscoping ruins it anyway.

The M16, PM-9, or MP5 require skill to use, I'll tell you that.

Basically, any gun that has a low fire rate OR high recoil (and by low fire rate, I mean less than 1 bullet/second). E.g. PM-9, pump-action shotguns, revolvers

  Since the beginning of my last prestige I have been running the M16 and I have to say it is "the" most bi-polar gun I have ran yet.

  Quietly wondering to myself how many new profanities I will invent trying to just reach the 500 kill marker with it.

In my opinion guns that take skill doesn't exist... what i think is that it all depends on the player... if you're a great player you will most likely do decent with any weapon..

FMG9 (akimbo if you really want to make things challenging)

PP90M1

P90 (rapid fire)

striker

it takes a lot of skill to make sure that your enemy is somewhere on screen before pulling the trigger and getting a kill with these guns. MW3 seems to have a really ambitious playing community too, because everyone is testing their limits with these guns every time i go online.

All these:

http://www.themodernwarfare3.com/mw3/weapons-list/

In that link...

MG36 looks like a G36

FMG has a scope

MP5 and UMP45 look the same

sub macchine guns look fat (pp90m1)

And 1887 looks like a rifle.

Akimbo FMG9's! Furrr R3AL!!!