If you're on PC the best things in general are:

Weapons

• BAR (Extended Mags + Rapid Fire, x4 isn't bad to remove sway if you camp areas - note that the other full auto AR's are solid too and so is the SVT, just the BAR is easily the most reliable atm)

• PPSH/Type 100 (Rapid Fire + Long Barrel, Extended Mags if you use Primed)

• MG-42 (FMJ + Extended Mags, Rapid Fire if you use Primed, mostly this is a Lookout loadout on the maps Lookout is broken on like USS Texas)

• Kar98 (FMJ + Rapid Fire, Ballistic Calibrations if on Primed which you usually will be, this is the best gun on PC if you are vaguely good at aiming)

If you have to use a Shotgun, the Combat Shotgun (Rapid Fire, Steady Aim, Long Barrel) is the best.

Basic Trainings

• Primed (reduced flinch is great, mostly on Sniper)

• Lookout (flat out broken on several maps, USS Texas and Saint Marie both have areas where you can literally wallhack people with this training.)

Espionage is alright too but it's not as strong as it seems to be considered on console, likely due to how fast mice can flick minimizing the need to pre-aim locations your opponent is coming from.

Mostly use Mountain division if you really want to level quickly and aren't doing WAR. If you want to prestige the fastest you should be partying in WAR though.

I'm sure on console this is probably pretty different and I can't help you there then.