Which are easiest to see, depends on a host of things, including your eyesight, the width of your front blade sight, how light or dark the sight is, etc. There is even a difference depending on whether it is a bright sunny day or a dark cloudy day.
For starters, you want a target of sufficient size to show where your bullets are hitting For sighting a new rifle at 100 yds, I generally make sure I have at least a full open sheet of newspaper behind a target with a round black bull of at least 6 inches diameter. (Some targets are available with the same size scoring rings but different size black circles.) After that, and beyond that, the target to use depends on the type of shooting you want to do. Sillywets, gongs, paper targets require slightly different size targets to shoot at. At a recycle center, I located some old heavy steel gears from farm equipment. Some are 1.5 inches thick and 8 inches round. they will even stand on edge. they make great cheapo silly wets. At 300 or 400 yds an old farm disc blade hung on chain works great. (they're a SOB to drill through though) they run 16 to 24 inches in diameter and are great for practice and approximate the black round bulls of distance paper targets.
