In order for the weapons to work at its best, equipping them with the suitable attachment is important. After playing this game for quite some time, and after some experiments and research, I have managed to figure out which attachment would be best for each guns. Below will be the attachments for each guns based on the information provided in the game itself as well as my own opinion which is based on my gaming experience.
SNIPER

Attaching a compensator and cheek pad to snipers such as AWM, M24 and Kar98K will be able to reduce the recoil of the gun. A compensator itself can reduce the recoil but the effect is not as obvious as when it is attached with a cheek pad as well. Attaching a cheek pad will greatly increase the stability of the snipers. AWM and M24 can both be attached with a magazine. Attaching an extended magazine to these two guns will not only increase the capacity of the gun, but also slightly increase the firing speed of the gun. As for Win94, attaching a bullet loop to it will not reduce the recoil but will increase the firing speed of this sniper.
As snipers are normally single mode only, heavy recoil does not make much difference. However, having an extended magazine is important as the reload time for snipers are quite long. Therefore, having a bigger capacity would save the player much time as they can reload after more rounds. Furthermore, an 8x scope is extremely important when using snipers. With an 8x scope, players will be able to have a clearer view which can help them to better head shot the enemies.
DMR

Attaching a sniper’s extended magazine on DMRs will be able to increase greatly their firing speed and capacity. DMRs can be equipped with both sniper’s magazine and AR’s magazine. However, sniper’s magazine would bring greater effect as compared to AR’s magazine. For DMRs that can be equipped with a muzzle (Mk14, SLR, SKS and Mini14), a compensator would be the most suitable muzzle to reduce the recoil of the gun. For DMRs that can be equipped with a stock (Mk14, SLR, SKS and VSS), attaching a cheek pad would greatly reduce the recoil. A cheek pad has a greater effect in reducing the recoil as compared to a compensator. The most suitable foregrip for SKS would be a light grip that can reduce the recoil of this gun more than the other foregrips.
Reducing the recoil is relatively important as DMRs have shorter fire rate thus making it important to improve the stability of the weapon. Having an 8x scope when using DMRs would be the best, however 8x scope is sometimes difficult to obtain. Players can use a 6x scope as well as it still have a further and clearer vision.
AR

Equipping a compensator on an AR will reduce the recoil. For Groza that cannot be equipped with a compensator, attaching it with a suppressor is also useful as it can be used to confuse enemy as player’s location will not be exposed easily. For AR’s that can be equipped with a foregrip (AUG, M416 and Scar-L), a light grip would be the best option as it can further reduce the recoil of the gun. Extended magazine can be equipped to increase the firing speed and capacity of the guns. Attaching a tactical stock on a M416 can greatly reduce the recoil of the gun. AR’s can only be equipped with 6x scope and below.
For players that having very good skill in this game, using a suppressor would be a better choice as it can greatly hide the player’s location. From my experience of playing this game, a fully equipped M416 would be the best AR. This weapon requires a number of attachment making it quite hard to fully equip it. However, due to the number of attachments equipped on this weapon, it becomes much more stable and powerful. Although AR can be equipped with 6x scope, I would rather use a 4x scope. This is because sometimes 6x scope is too big and makes it difficult to aim at enemies during “emergency” situations.
SMG

For UZI, equipping a stock does not reduce much of the recoil. However, if it is equipped with a compensator, the recoil of UZI will be reduced. A Tommy Gun can only be equipped with a vertical foregrip and a suppressor. Equipping the suppressor can help to avoid location exposure and not to reduce the recoil of the gun. The recoil of this gun can be reduced by quipping a vertical foregrip. Attaching an extended magazine to these guns will increase the firing speed and capacity of these gun. Both UZI and Tommy Gun cannot be equipped with a scope.

As for UMP9 and Vector, attaching a light grip will reduce the recoil greatly. Nonetheless, equipping a compensator and tactical stock (for Vector) will also reduce the recoil, but not as much effect as compared to the light grip. Similar to the UZI and Tommy gun, an extended magazine can increase the firing speed and capacity of these guns. Both of these SMGs can be equipped with scopes 6x and below. However, due to the range of these guns, using a 4x scope would be good enough as SMGs bring not much damage to enemies that are at a far distance.
Out of all the SMGs, I like to use UMP9 the most. This is because this gun is very stable and is powerful for close range. I like to equip UMP9 with a vertical foregrip and any muzzle and magazine. Although equipping it with a compensator will reduce the recoil, it is not really needed as this gun’s recoil is quite easy to control. Other than that, a red dot is normally equipped on this weapon. Red dot is usually used in close range battle. A 4x scope is hardly used, but when it is used it will be for close-to-mid range battle only.
Shotgun

Equipping a bullet loops on S1897 and S686 can only slightly increase the firing speed of the gun. A choke does not make any difference whether equipped or unequipped. It can be said that there is no difference for these two guns with or without any attachments. However, for S12K, attaching it with a compensator and extended magazine can reduce the recoil and increase both the firing speed and capacity of this gun. As shotguns are designed for close range only, equipping S12K with a red dot or holographic sight is good enough. However, players can also equip it with a 2x or 3x scope for a slightly further range.
From my gaming experience, shotgun is normally used in the beginning of the game. There are two reasons why. First is because shotguns can be found anywhere in the battleground. Second is because shotgun is very powerful when used in close range. At the beginning of the game, players usually use a shotgun just in case they run into enemies while searching for their desired weapons. Shotgun is quite hard to use as they have long fire rate, small capacity and has only single modes. Honestly, after playing for such a long time, I still have not mastered the skill of using shotguns. Other than it being difficult to handle, another reason is because it is only used for a short period of time in the game.
Pistol
As for Pistols, equipping them with or without attachments does not make much difference. Most of the pistols can only be equipped with a muzzle and magazine. Equipping a compensator for pistol does not increase the stability of the gun. An extended magazine can help to increase the firing speed and capacity of the pistol. However, as Pistols is only used when players cannot find any other guns, looking for attachment is not necessary as they will be replaced as soon as another gun is found. As Pistols can only be used for very close range, using a red dot is good enough.
Similarly to shotguns, pistols are used at the beginning of the game for “emergency” situations only. There is not much skill for using pistol as they are all relatively easy to handle. However, pistols are not powerful at all which makes them the least favourite weapon.