Well it's like everyone has a butt, and opinions are about as plentiful, so here are my thoughts on zeroing a rifle. The first thing that comes to mind is what cartridge are you shooting as well as the scope you are shooting. Obviously something that is distance limited like a .22 LR it's a pretty easy decision. A good quality 3 x 9 fixed scope does the trick. Since the parallax is not adjustable and set at the factory for 100 yards that would be my go to, and where I zero my .22 rifles. In my case there is a Vortex Crossfire II 4 x 12 x 50 with a 1 inch tube since there is not going to be a need for a lot of adjustment. This same theory goes for traditionally short range rifles in 30-30, 35 REM and the like.
For longer range rifles, like the 270, 308, 30-06 and anything similar that can be shot at reasonable ranges beyond and be far away, it's a different story altogether. If you are only going to shoot at 200 yards or less then all you need is, like the .22 LR simply a 3 x 9 or 4 x 12. Despite the parallax on these rifles is factory set to 100 yards (there are some that do have a parallax adjustment if yours has it, set it and use it) On these rifles I sight them in at 200 yards. On most hunting rifles if sighted in at 200 yards then a shot out to 300 yards can be done simply by holding dead on in the kill zone and squeezing the trigger (new terminology is pressing the trigger) and most hunting ammo will hit with no more than an 8 inch drop from where you are aiming when the rifle goes off. In this case a 200 yard zero essentially gives you a shot within the kill zone out to 300 yards.
For Long Range rifles, and I will let you make your own determination as to what you consider Long Range then the scope requirements change and get more expensive. If you are going to shoot at longer ranges regardless if target shooting or hunting a good 30 MM or 36 MM scope is recommended. A good one that I have used in the past is the Vortex Razor HD now in its third generation. It has a 120 MOA vertical adjustment range that is needed for extreme long range shooting. With scopes with adjustable turrets always zero at 100 yards. This is a good place to come back to after taking a long ranges shot. Using a scope like this requires shooting it at all the ranges you expect to shoot at and marking the elevation (primarily) for the scope setting at that particular range.
Well there you have it. My thoughts on what scope and how to zero it for just about any kind of shooting. Let the discussion begin.
I can never remember the conversion factor though so when I use my rangefinder, set to inches, it takes forever to get on target. It goes something liked this: get distance in inches, naught from naught carry the naught, clickity click, ding. Shoot. Dang it. Range find. Rinse and repeat. 
