Scopes can be used for a wide variety of usages ranging from competition shooting, to sniping, recreation, to the more commonly utilized role of hunting. What defines the best scope for you is generally a combination of what fits, what it will be used for, and price point.
I'm going to use Leupold as an example since they are widely available, quality products, and offer a wide range of pricing options.
Some common options and terms defined:
Base Power - The lowest magnification the scope offers.
Fixed Power - This is a scope that has a fixed magnification. It is not adjustable.
Variable Power - This is a scope that has a variable magnification. It is generally expressed as the base power 'x' the highest power. A common example is a 3x9; meaning it has a 3x base magnification, and a peak magnification of 9x. A twistable ring on the scope adjusts between the range.
Eye Relief - This is the distance from the rear of the scope to the focal point of the scope. It is the point where your eye will obtain the maximum field of view when looking through the scope. At lower magnifications there is a much wider area that gives a full picture in the scope, and with high magnifications (generally 14x or higher) you have a very narrow area.
Objective Lens - The large lens at the end of the scope.
Ocular Lens - The small lens nearest to your face when you look through a scope.
Objective Diameter - This is the maximum size of the objective lens. Common sizing ranges between 30 - 54mm. It is almost always expressed following the power range; ie. 3-9x40, meaning a 3x to 9x variable magnification with a 40mm objective.
Tube Diameter - This is the diameter of the tube, the skinny middle part. Tube diameters are mostly 1" (25.4mm), but are found in sizes ranging from 30 - 34mm in scopes designed for low light, European market, or long range shooting. Be forewarned that scopes with a 30mm, or larger, tube may limit your choices for mounting rings.
Turrets/ Adjustment Knobs - The turrets are the nodes found at the center of the scope for adjusting the windage, elevation, illumination, and sometimes the parallax. There are a variety of options available in turret choice.
MOA - Minute of Angle (or Arc). It's a geometrically defined amount used to express a cone of fire. 1 MOA is 1.047" at 100 yards, but is generally held to be 1" at 100 yards for simplicity.
Click Value - The amount each individual click of the adjustment knob moves the point of impact. The far majority of scopes are in MOA, although European scopes like some Schmidt & Bender's, or Leupold's Euro line, are in metric.
Elevation - Quite plainly, the up and down adjustment. Adjusting this alters how much elevation you give the barrel relative to the scope, hence it's name.
Windage - Again, quite plainly, the right and left adjustment. It's name refers to applying lead through the scope to account for wind drift.
Parallax - This is an optical effect involving two focal planes being out of sync relative to your eye. Place one finger at full extension in front of your face, and using your other hand, place a second finger midway between your face and extended hand. Now without moving your hands, move your head side to side. You will notice that the middle hand appears to be moving at twice the rate of the far hand. This is what parallax explains, and when looking through a scope, it describes why the crosshairs move around when you move your head slightly.
Scope Reticle - This is the crosshair image. They are found in a wide array.
Illuminated Reticle - This simply means that the center of the scope rectical is illuminated either by fiber optics (Trijicon), or more commonly, a battery. They are variable in power on a seperate rear mounted adjustment knob.
Bullet Drop Compensator - BDC is a pre-defined amount of bullet drop based upon a particular load for a particular caliber. It has a series of dots, or an adjustment, that allows you to have multiple zeroes rather than a traditional hold-over.
What to Look For / Things to Ask Yourself:
- Does this scope support my intended purpose. - Does this scope physically fit my rifle.
- Can I find rings that fit this scope tube diameter for my rifle.
Is the adjustment friction type, or click type?
Some notations regarding the two. It is FAR easier to adjust a click type rifle properly, which means you aren't coming off the rifle. With a friction type you need to visually check the adjustment which means you are introducing parallax error to your next shot, making finite changes difficult.
What do I personally recommend?
I believe the click-type 1/4" MOA adjustment is the most widely available, and best solution out there for 99% of the shooting population. Frankly 2 MOA and 1 MOA just sucks, and 1/2" MOA is better suited for tactical shooting (200 yards or less). 1/8" MOA is rather finite, and better suited for long range shooting (400 - 500 Yards+). Generally accuracy at those ranges is less due to scope zero and more to do with ammo and ambient corrections.
1/4" MOA Click Type Adjustment Knob
Pop-up Turrets with Clear Markings for Field Zeroing
Hunting - Hunting is a wide topic. Generally in Texas most hunting is done within 200 yards on small game. Most hunters will prefer, and utilize, a low to medium power scope; base magnification of 1.5 - 3, and a peak magnification of 9 - 14. Lower magnification gives you a wider field of view, which comes in handy when hunting pigs. Daylight hunting gives ample light for just about any objective size and tube size, in fact the standards are 30 - 40mm objectives and 1" tubes. For lower light conditions, or for just more light in general, you will want to go with a 50mm objective, and possibly with a 30mm tube diameter. Bear in mind that the larger objective increases the physical size of the scope, and can cause mounting problems. At 100 yards, a 6x to 9x scope will be sufficient for accurate shot placement within a 3" x 3" window. Beyond that, you may opt for a 12x or 14x peak magnification. Since you more than likely will be storing a hunting rifle for elongated periods, and crawling around through brush and other field conditions you will want to have a scope that has caps over the adjustment knobs to prevent them from being adjusted by contact.
Long Range Shooting / Competition - This kind of usage requires high magnifications (20x and above) with thin reticles designed not to shroud the target, or marked with mil-dots to allow a proper hold-over for example. Typically they will have an elongated objective shroud (called a sun shade) to shield the image from barrel heat induced mirage and image noise. Items like adjustable parallax, capless target turrets, etc. are common. For variable range adjustments you will want the scope mounted as low as possible to avoid mechanical difference between the bore and scope axis.