There are multiple ways to improve the way Payday 2 looks, using GEM is a good place to start, using an injector such as SweetFX will also help.
The game utilizes a chromatic aberration filter (color separation) which blurs textures and increases the effects of aliasing. The GEM project's Payday 2 package includes a utility to delete this filter from the game's data files, but the change may be reverted by game updates.
BLT-Hook is essential for having multiple mods in Payday.
Pocohud is an incredibly popular fan made HUD mod. While not essential, it can dramatically improve your experience.
Ambient Occlusion compatibility
Ambient Occlusion usage
Ambient Occlusion setting