What do you mean with "best" ? You know that the guns are balanced around each other? Pick the one according your playstyle, nobody will be able to tell you with which weapon YOU will have the most success, that's what the "trial weapon feature is for", try em all out and then decide.
Should have posted this in the LA section. But I'll help you out. For CQC, I prefer the Serpent with a 1x/2x reflex site. It tears up anyone who's close to you. You can equip the laser site for better hip fire, but I prefer to keep that off because I like to be stealthy. Medium/long range, it's a tie between the Pulsar C with HVA and a 4x scope and the Thanatos with slug ammunition and a 4x scope. Both of them do the job well, so it's all about your preference. You're going to get a lot of mixed responses, it all boils down to personal preferences.
Yea I assumed there wouldnt be "1" answer, just dont want to buy something useless ^^ Thanks for the responses guys. I might go CQC since my Heavy is built is a mid/long range, still not sure yet though, just want something thats good in Tech plant fights.