There are many good ones.
1. UMP + FMJ, M9 + tactical knife, throwing knife, stun grenade, Lightweight pro, Marathon pro and commando pro
2. ACR + Grenade Launcher, semtex, claymore, one man army, danger close pro, commando pro
3. Riot Shield, RPG-7, throwing knife, stun grenade, Danger close pro, Scavenger pro, commando pro
4. Intervention + FMJ, SPAZ + FMJ, claymore/semtex, stun grenade, steady aim pro, stopping power pro, sleight of hand pro.
There are many others, but that's all i can think of.
The ACR or the AK-47 they are excellent
It depends on what are you good with.
ak47
Yes, it is the final gun to unlock in the game.
You get kills with the gun you want to unlock attachments for. If you see your challenges it will tell you what to do to unlock the attachment.