The best sniper rifle in the game is probably the badboy I found recently:
Instigating Gauss Rifle (fully modded including suppresor)
Base damage with Rifleman and Bloody mess perks: 463
Sneak attack damage with Ninja, Mr. Sandman and follower perks: 590%
Instigating: double damage to enemies at full health
Sneak shot at an enemy with full health = 463 * 5.9 * 2 = 5463 dmg
I have not yet seen an enemy that would survive the first shot to the head and I am playing on Survival difficulty. If I get caught out of stealth however, shotgun or assault rifle is the way to go or the death comes swiftly (END 1).