On Console and PC, it's the Sniper Rifle.
On Console, it does around 162 and about 345 crit damage, because i'm Melee (Tank) on Console. On Console, I'm using Beetle Armor with Shell.
On PC, it does around 333 damage, 445 while stealthed, and about 900 - 1035 crit damage, because i'm Ranger on PC. On PC i'm using Shroomite Armor with Bullet Headpiece (i can't think of the name).