Quick scope actually is kinda hard to learn and takes practice along with skill, but yes it definitely can be very effective once u get good at it and use the 1 shot L96.
But if you're looking for the best no scope sniper, then pretty much any semi-auto sniper would be good, or the L96 if u can land the first shot. In sniper no scope, the closer u are to the enemy, the more chance u have of hitting them. Try to run a straight line towards the enemy when you fire, it seems to make the bullet shoot a little straighter and closer to the center of screen, or u can do it the hardcore way, walk up and stick the barrel in the other guy's face then pull the trigger, but that's just a little more risky.