Agility, Perception, guns, sneak. If hardcore throw in a few points to survival.
Here is a useful archetype, as suggested by the game guide.
Tag: Guns, Sneak, Repair
Traits: Four Eyes, Small Frame (If you wish to be a stealthy Sniper), Trigger Discipline.
Perks: Sniper, Commando, Finesse, Center of Mass, Better Criticals, Rapid Reload, Weapon Handling, Silent Running, Hand Loader, Confirmed Bachelor, Bloody Mess, Hunter, Entomologist, Robotics Expert, Travel Light, Living Anatomy.
Weapons: Any Rifles, Scoped rifles and Sniper Rifles.