Great info. Here's some additional information given by a fellow hacker:

Automatic
The best automatic gun to choose is the SFCR-HW. It has the highest damage and best accuracy.

The AK-47 and ATS-86G come in second doing the same amount of damage, but 2 times less than the SFCR-HW.


Sniper
For the best sniper choose the M87ELR. It may have slow reload, but it's got the best damage for a gun, it's practically one shot kill every time. Did you know it's the strongest primary weapon available in the game to non hackers? So what if it isn't as loud as the L96AW...

The L96AW does come in second in terms of damage though. It's not as powerful as the M87ELR, but does reload faster. This is helpful to new players, but in terms of experience, the M87ELR is the gun to choose.


Secondary
As you may already know the SPZ-15 does the most damage, but did you know the regular crossbow (Not the repeating crossbow) does the exact same amount of damage as the SPZ-15?

The second best secondary would be the M4-90 or the SG-7, both of which do the same amount of damage.

The third best secondary would be the repeating crossbow.

The forth best is the GMP.


Grenades
HE's actually do the most damage believe it or not.


AT4 or RPG
They both do the same amount of damage which is 5 times more than a grenade.


Attachments
The M203 HE does the most damage. Shotgun does the least out of all the attachments.


Random Fact
Did you know that a bomb does more damage than One Shot Kill?