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.
This is my best class I use it about 80% of the time.
Class: Best Sniper
- Primary: Barret .50 Cal with Suppressor and kick
- Secondary Primary: MK. 14 with Red Dot Sight, Suppressor and Attachments
- Lethal: Claymore
- Tactical: Portable Radar
- Perk 1: Scavenger Pro
- Perk 2: Overkill Pro
- Perk 3: Marksman Pro
- Strike Package: Assault with Predator Missle, Attack Helicopter and AC-130
- Death Streak: Hollow Points
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.