Enemy snipers are a major pain in The Division. But managing enemy snipers doesn't necessarily mean killing them, but suppressing them so they can't drop your teammates. Also, enemy snipers switch to a lesser weapon when rushed, so suppressing them to allow a teammate to charge them is a great maneuver. Here's the key to building a good sniper.
Smart Cover with the Recharger mod
The Smart Cover skill, from the Security tree, lets you reinforce an existing piece of cover, which does two things: increases outgoing damage and decreases incoming damage. That's about perfect for the sniper, though anyone on your team using the cover gets the same benefits. Equipping the Recharger mod does double duty: anyone on your team using the Smart Cover not only heals but gets a reduction in cooldown time for their skills.
Sticky Bomb with the Proximity Fuse mod
I feel like I should say Pulse, which is extremely useful for the entire team, but it's also the very first skill you receive and sometimes you just want something new and fun. I'm going with the Sticky Bomb launcher, mainly because of the Proximity Fuse mod, which turns your bomb into a proximity mine. As a sniper you're going to spend most of your time peering through a scope at enemies far, far away, which can leave you vulnerable if some creep slithers up to your vantage point and starts whomping on your head. Placing a sticky mine in the vicinity, especially in a doorway or on a staircase, can blast the baddie before he reaches you, or at least let you know he's coming.
Signature Skill: Tactical Link
Unlocked from the Tech tree for 1,000 points, this skill increases damage and critical hit chance, very important for taking out highly durable and distant enemy snipers.
Let's start in the Security tree. Stopping Power increases headshot damage on a target you've suppressed by 25% for 10 seconds, and it's hard to think of something more useful against enemy snipers. A close second is Steady Hands, which reduces your recoil by 25% for 10 seconds upon entering cover, perfect for making sure your first couple shots hit their mark. One is None gives you a 50% chance of retaining your bullet (somehow) if you headshot an enemy. On the Tech tree, Tactical Advance increases your weapon damage for 10 seconds after you've moved from cover to cover, and the further the move is, the bigger the weapon damage bonus.
Key Stat: Firearms
I guess that's obvious: you want your rifle to do as much damage as possible, and lurking at the back of the team hopefully means you can get by without much Stamina. Electronics is important as well, as it'll keep your Smart Cover up and functioning for longer.