What is the best peep sight made and readily available now?
What about a replacement front as well? Or is there a peep that works well with factory front sight?
Which sights adjust the easiest?
What is the application? "Best" is a slippery target.
For a receiver mount on a lever action or general hunting rifle the Williams FP is the most precisely and easily adjustable and widely available for a variety of rifles.
For a set and forget an XS or a Skinner will both serve well and are extremely strong. The Skinner will generally sit lower and both companies offer replacement front sight blades, the Skinner flat black .1" wide front blade makes a nice sight picture, the XS sights almost all have a white stripe down the blade which some folks swear by. The Skinners generally cost less for the basic sight and front blades and has very personalized customer service, XS can be kind of a pain to get a hold of sometimes; my experience with them varies from answering the phone right away to ignoring repeated phone calls and emails. Andy at Skinner will get back to you with a personalized email generally the same day you send one, I've had replies from him even on a Saturday.
If you can do simple algebra (X" over sight radius = Y" at 100 yards) you can adjust the XS and Skinner by thread count, although the Skinner windage is purely guess and by golly without a measuring device of some kind. They work, but a Williams has fairly accurate clicks for both elevation and windage.
If you're looking at a tang sight I can say with some certainty to stay away from the Marbles - it wobbles in the upright position. A soule sight of some kind is extremely precise but you pay for it. Various small makers offer those and you can start in the $200 range and go all the way up to $500 or more. I have a Lee Shaver "economy" Soule - $260 - but even at this level it offers extremely precise vernier adjustments.
Strong, set and forget - XS or Skinner.
Precise adjustments - Williams
Really precise adjustments - Soule type.
