Heavy Weapons and SWAT give you the most for your money.
The RPG is painfully slow and takes forever to reload, but it's good (for now) in close up battles. The dynamics may change if/when Chapter 2 is fully rolled out. Devs have hinted at nerfing it somehow so that people can't spam it at the ground near their feet and deal splash damage to everyone around them.
The AWP is second to none if you enjoy sniping. 2 round mag with somewhat slow reload but rounds deal 80 damage, effectively knocking someone with armor if you can land both shots. Quickly switching to the shotgun or Deagle after the first shot can be lethal if you have good aim.
The M4A1 deals 10hp damage vs the SCAR's 15, but it makes up for it with a faster fire rate and less recoil. Not to mention you get the SWAT van (slow and clunky to handle but it can hold your squad, up to 6 I think, and has bulletproof tires) and uniform.