Some would say Anschutz, others would say Remington, others would say Walther.
Too broad to answer. At what range? Rifle, rimfire, pistol, revolver? In general, a heavy barreled target rifle (most are custom builds) in one of the 6mm target calibers would be the most accurate to about 600 yards, but other rifles such as the .338 Lapua will be more accurate at 1500 yards.
It is often accepted the Heckler and Koch PSG-1 is the most accurate sniper rifle in the world.
The most common target air rifle pellets are made of lead, although other materials such as tin, steel alloys or even plastic are used for pellets.
Anschutz and Remington 700 are regarded to be most accurate rifles in the world. NOMAN
From a benchrest.
Custom built
Impossible to answer as asked. Rifle, pistol, shotgun? What does accurate mean to you?
It is generally accepted the the pellet rifles used in the Olympics are the finest made. Look up rifles made by FEINWERKBAU WESTINGER & ALTENBURGER
You have to work up the most accurate load for your particular rifle. Each rifle will respond differently to changes in bullets, powder charge, seating depth, etc. What is an accurate load in one rifle may shoot poorly in another and vice versa.
Impossible to answer as asked. Factory or custom?
Depends on the target. Generally, but not always, a telescopic sight.
You will have to define "sniper rifle". There is no meaning to that term, other than an accurate scope mounted rifle. That includes most hunting rifles.