For me I think the most useful is a fully-modded sniper rifle. It's silent, very light and packs a punch even without the sneak crit. I love how the Ratslayer looks, and the cheapness of its ammo and the fact that it's bolt-action and has little recoil. The damage is a little too low for late-game enemies though, especially if you're like me not a good sniper and don't always get sneak headshots. The anti-materiel rifle is lethal, but the recoil is ridiculous, and it's a little too slow. Not to mention the ammo is so expensive. I don't generally use the AMR until the final mission (love seeing Lanius flyyyyy), I prefer the sniper rifle even when hunting deathclaws since it doesn't give away my location. Essentially a good compromise between power and speed/cost.
(Should I include the marksman carbine? It's scoped and a rifle, but I see it as more of a mid-range weapon rather than a 'true' sniper rifle.)