It's a matter of opinion for a lot of classes. Each weapon has pros and cons.
Just find what suits you the best, try some different things. If you like to have a weapon that is good on any range, I'd suggest the MP7. If you want a weapon to do good in Ground War, hands down P90, and for rushing like a complete beast and not caring about anyone or anything, a PP90 is probably your best fit. Just pick whatever you like.
i don't reccomend playing game any more on multiplayer past week or so i've ran into so many groups wth 60 kills 10 deaths and spawn with vests on so it snot worth playing any more
mk14 guns a beast also ive been runnng about with just the five sever pistol today had some fun and still always go positive.
Best Assault Rifle - ACR - I usually go with Silencer and Extended Mags
Best SMG - MP7 - I usually go with Silencer and Extended Mags
Best Sniper - Not sure - I don't snipe I'm horrible at it.
This is my best class I use it about 80% of the time.
Class: Best Sniper
The .50 Cal is for long range engagemants, the Mk. 14 is for those moments of panic. The claymore and portable radar are your best choice for watching your back, don't rely on the Claymore to defend you fully because some people run the Perk 3: Stalker Pro. This is by far my fav class. I have used it since I got the Mk. 14 at level 60. I'm currently prestige 8 rank 78.