Sorry for the slow reply. It took me a while to get my thoughts together. I'll break this up into three posts.
I tend to look for basic performance rather than gimmicky features, and avoid getting tied to any particular brand. I tend to prefer manufacturers that have a practice of designing and testing their own optical systems. What follows is a summary of my observations as both a developer and user. I divided scopes into four broad groups, depending on main design features and price.
The bottom. There isn’t anything out there that I would recommend below $300. These scopes have large, polycarbonate bodies and conventional (non-HD) doublet objectives. Polycarbonate is used because of it’s impact resistance and high temperature rating. Otherwise, it’s not a good structural material for precision optics.
By today's standards, non-HD glass is pretty poor. Color fringing is noticeable everywhere in the field of view and the image goes out of focus about half-way toward the edge of the field of view. Eyepieces are small and have a very limited field of view and eye relief. They usually have rubber eyecups that roll back to increase eye relief. This is a useless feature, as the eye relief is nowhere close to being long enough for most eyeglass wearers. Lens surface quality is poor, resulting in low contrast images. Their use is limited to non-critical viewing tasks at short range. They generally fall into the “better than nothing” category. If you have to start here, I recommendation getting something with a good warranty.
Low price range: $300-800. There are lots of options in this price range, but you will likely want to trade up within a few years. These scopes are usually made in China at the low end, and the Philippines at the high end. These products will have variable quality control and return rates will be higher than average. Some products in this price range are really just cheaper product designs with upgraded lenses, so look closely at design details.
I recommend looking for a 50-65 mm doublet objective lens with HD, ED or XD glass (all seem to be synonymous terms), and fully multi-coated optics. These scopes will have a polycarbonate or fiber reinforced polymer (at the high end) body and a small, non-removable, short eye relief eyepiece. The objectives tend to have a long focal length, which makes the scope body long. The use of polycarbonate makes the scope body even more bulky. The big 80-85 mm scopes in this price range tend to be really LONG. The use of polycarbonate for the tripod mount is very common. Polycarbonate is not a very stiff material. The scope flexes rather easily, causing the image to shift and shake whenever the wind picks up or the scope is handled. This is especially the case for the rotating mount on angled scopes.
You can expect the mechanical controls to have loose tolerances and use lots of plastic parts. I find that zoom and focus adjustments eventually get loose and twist-up eyecups won’t stay in position. I discourage people from going to the higher magnification, 80 mm or larger objective scopes in this price range. The short eye relief eyepiece usually does not allow effective use of a scope at 50X or higher magnification.
The Bushnell Legend Ultra HD is typical of scopes at low end of this price range. This scope is adequate for many applications but not great for anything in particular. Prolonged use can cause eye and neck strain and headaches. The reason is the need to constantly shift your eye to prevent the familiar bean-shaped obscuration as you look around the field of view. Scopes at the high end of this price range have somewhat better ergonomics. Notable examples include the Vixen Geoma II ED and Vortex Viper HD.
Mid price range: $800-1,200 (50-70 mm), $1,000-1,500 (75-85 mm). The higher quality scopes in this price range can provide years of good service. Many of them are still built in Japan and the US and have excellent quality control.
In this price range I recommend looking for scopes with aluminum or magnesium alloy frames or bodies, eyepieces with long eye relief, and HD/ED/XD doublet objectives. There are a lot of polycarbonate body scopes competing in this price range on the basis of extra features, and I recommend against them. Aluminum and magnesium alloys have a stiffness to weight ratio that is nearly 10 times that of polycarbonate. Metal alloy construction in the tripod mount and rotating ring on angled scopes is a big upgrade in this price range. Metal scope bodies are stiffer, more compact and usually lighter than comparable polycarbonate ones. This means the image will shake less during wind gusts and when the scope is being handled to adjust magnification, zoom, etc. High temperatures are generally bad in optics. Metal conducts heat much better than polycarbonate, so the internal optics will not have hot spots when the scope is exposed to direct sunlight.
The best scopes will have somewhat larger eyepieces that offer improved eye relief and field of view. While fatigue can still occur after prolonged use, these scopes are a big improvement compared to the low price range. They will have good transmission but colors will generally not be accurate. Image contrast will be good but not great. Be aware that HD/ED/XD doublet objectives have limitations. They produce a sharp image that is largely free from color fringing near the center of the field of view. This “sweet spot” will cover about ½-⅔ of the field of view. Beyond that the image sharpness will degrade due to focus shift.
Mechanical design and durability will be a big step up from the low end scopes. Zoom and magnification adjustments will be more precise and twist up eyecups are durable. Digiscoping options will be limited in this price range. At best the manufacturer will offer a digital camera mount that clamps onto the scope. Switching back and forth between the eye and camera is cumbersome. Extra features that are nice but not necessary include hydrophobic lens coatings to help shed water, optional eyepieces, etc.
In my opinion, examples of good 60-65 mm scopes in this price range include the Pentax PF-65 ED, Kowa TSN-663/664 and Brunton Eterna. Even though it lacks a removable eyepiece, the Leupold Gold Ring HD 60 mm is worth mentioning because it’s so compact and durable, and the looong eye relief allows it to be used in almost any viewing position. The digiscoping attachments in the Gold Ring HD 60 mm kit depend on using fixed focal length DSLR lenses that fit the few thread adapters provided. Otherwise, they work very well. Note that these scopes have been on the market for a long time, and they offer high value. The 80-85 mm versions of these scopes are also good. For hunters, the larger objectives are warranted when the spotting during the twilight hours around sunrise and sunset. The larger objective also buys an extra f-stop or two when digiscoping.