AK5C lets you have (basically) a laser AR on any class. AWS can be unlocked by a challenge rather than progression, and is one of if not the easiest LMG. AK-12 will hold you over for as long as you need, and still makes a good weapon when you reach high levels M416 though will basically let you stay on target without moving a single stick. So easy to use. G36C is kinda underappreciated IMO, but it's a beast and easy to use. If you like the gun IRL like me, it could easily become your favorite. MPX is a pretty beastly little PDW, but you're not gunna get much from PDWs in general. You're better off with the carbines on your engineer honestly, and they unlock relatively easily. Those are some of the low recoil, keep your s**t on point guns you'll like if you're coming from CoD.
When I started though, I hadn't played a multiplayer FPS in a while. And my favorite from the start was the Scar-H. If you want lower ROF and high damage models try that, the Bulldog AR, the UMP-45 PDW, the PKP Pecheneg LMG, or the ACE 52 carbine.
Hmm, depends on your style of play. I would say the best carbine is the ACW, if you can get past the recoil. If you have SA, I recommend the F2000 or M16A4 if you don't. As for sights the Kobra sight is in it's own league, There's not need to use any other sight if you're not playing Recon.
AR- M416/Scar/Ace 23/Bulldog SMG- AS val/UMP Carbine- Ace 52/ACR/ACE 21/ TAR-21 Shotgun- Spas-12/Saiga-12/ That one that looks like the AK-12 Sniper- Intervention/M98 DMR- Scar/ACE 53 LMG- M240B/MG4/M249/PKP Secondary- Desert Eagle