Rainbow Six Siege doesn't have a lot of weapon customization options as compared to other games on the FPS market today, but that doesn't mean what you put on your weapon isn't important. Weapons are going to perform pretty differently depending on what attachments you put on, and that can make a huge difference both in how a match plays out and what engagements that you take.
Let's take a look at different barrel attachments first.
The compensator increases weapon stability by 17.75% by making the weapon kick more vertically than horizontally. This makes it a good fit for weapons that have a substantial horizontal kick since the compensator makes the recoil more predictable, and managing vertical recoil is easier than managing horizontal recoil.
11 FLASH HIDER
Besides dampening the flash of your weapon and in turn making it easier to keep your target in view, the flash hider also reduces first shot recoil by 37.5%, reduces centering time by 30%, and improves weapon stability by 5%. This is a nice middle ground barrel, with the added benefit of the slight dampening of your muzzle flash.
The suppressor makes your shots harder to hear and locate, hides your bullet trail, and removes the indicator on your opponent's screen when you hit them that shows where your shots came from. Aside from that, the suppressor also reduces your damage output by a little bit and does absolutely nothing to help with your recoil.
9 MUZZLE BRAKE
The muzzle brake reduces the first shot recoil and the centering time each by 45%. In Siege, where most fights are decided in less than a second, those first few shots are absolutely critical in taking your opponent down. This makes the muzzle brake one of, if not the most, valuable barrel attachments in the game.
8 EXTENDED BARREL
The extended barrel is supposed to extend the range at which your weapon will do greater damage, so reducing the damage drop off. But in Siege, most fights are inside a range where this doesn't really provide much of a benefit whatsoever. The extra damage is only usually going to make a one-bullet difference at significantly high ranges, and the likelihood of hitting those shots goes down because you've put an extended barrel on your weapon instead of a barrel attachment that reduces recoil. So, all in all, it's not usually much of a benefit to using an extended barrel.
7 VERTICAL GRIP
The vertical grip reduces the vertical recoil on your weapon. Just about every player in Siege should and does use the vertical grip on most weapons that it's available on, simply because of its ability to make it a bit easier to land your shots. While vertical recoil is easier to compensate for than horizontal, Siege is a game of milliseconds and millimeters, so any improvement to recoil control is valuable.
6 ANGLED GRIP
The angled grip reduces the amount of time it takes to aim down sights. This might seem like a pretty good benefit, and in games like CoD or Battlefield, it would be. But in Siege, most of the time that you get into a fight, you're already going to be down sights before the bullets start flying. That reduction in time isn't going to help you out much in most scenarios, but that reduction in vertical recoil from the vertical grip might.
5 LASER SIGHT
Laser sights reduce hip-fire spread, and also have the added benefit of giving your location away to anybody that's paying attention. Seriously, on almost every automatic weapon in the game, laser sights should absolutely be avoided. It is absurdly rare that you should even be attempting to hip-fire somebody with most weapons in Siege. That said, there is a case to be made for using them on shotguns, as it reduces their spread, making them a bit more lethal. They should also be used on shield Operators pistols since you'll be hip-firing with them pretty regularly.
4 ACOG SIGHT
ACOG sights are the preferred sights of just about every single person that plays Siege, and for good reason; they zoom in, making enemies loom larger in your sights and making their heads easier to hit. Certain situations, like when an enemy surprises you and you're suddenly in a close-range gunfight, the ACOG can be a liability, but most fights in Siege are at a distance that an ACOG is going to be a huge benefit.
3 HOLOGRAPHIC SIGHT
The rest of these sights is mostly up to preference. The holographic sight has some pretty nice visibility and a circle reticle, making it pretty easy to fit someone's head inside that circle and get that quick kill. It's also worth noting that the holographic sight can look a little bit different depending on the Operator you've selected. Most Defender's don't have access to ACOG's, so getting comfortable with one of the other sights is probably going to be a necessity. Try each of them out for a while, and give each of them a fair chance.
2 REFLEX SIGHT
The reflex sight is another zoom-less scope, and it's one of the smaller scopes possible. There's not a lot of the scope that impedes your view, meaning even if someone isn't right in the middle of your crosshairs, you'll still have a pretty decent shot at seeing them. The reflex sight is another sight that can look a bit different depending on the Operator; the Spetznaz version of the reflex sight, for example, is quite a bit different than the reflex sight you would find on Castle.
1 RED DOT SIGHT
The red dot sight is the last of the low-zoom scopes. While it isn't quite as intrusive as the holographic can be, it isn't quite as slim as the reflex. The red dot sight can be used on just about every Operator in Siege and looks more or less the same on every single one of them.