It is difficult to describe what to look for.
Basically want it to be full vinyl, heavy weight. Good glass, good workmanship, Low E, all the hardware heavy duty. Should have bottom strip that seals the angle formed with the window framing, this will be separate from the window and snap into a groove in the bottom. Allows for a seal and better looking appearance because most older windows have a slope to the stool area. Top should have an adjustable cap that allows about a 3/4" tolerance to fit the window in the vertical plane.
You also want the flanges around the window to be a full U shape. This will vary from manufacturer. Some only provide a flange on the exterior side. The better window will have a double flange, both inner and outer, this forms a U shaped channel that is easy to pre-insulate before installation. Plus it makes the overall window stronger and stiffer, you get two surfaces to seal with caulking.
In general a bigger window like 60" and above will be quite heavy and take a bit of grunting to lift. The poor windows will have a noticeable difference in weight and the amount of vinyl in the framing. That is one of the major difference between windows. Plus the poorer windows handware in general is also smaller and more cheeny. Things like the lock mechanisms, etc.
Your typical better grade window will cost something like $180 on average (contractor's price). Each window will have a sticker with a serial number, its dimensions and be registered with the manufacturer. Replacement parts, glass, etc can be gotten using that information.
I don't think much of most of those window replacement companies. Your typical mark up on a window is ~100% for the installation. They try to make a big mystery about the entire process and pretend it is difficult, play games about discounts and do whatever to fool you. It is a fairly easy buck but the markups are justified in a way not from all the work to install them but because of all the time and running around prior to actually getting the windows. So something like $300 per window for the better grades might be a fair price if a contractor installs it. Do not try to judge window quality from one of them lil demo window things the salemen carry around. You need to see a real window to judge.
From the contractor side of it, you want to bag every job. Quotes cost time and money. Nothing is free, especially if you have to go thru the process of doing all the measuring. Explains a lot of the games that happen, the customer is not always right. You tend to not give as high a price break in older houses. If there are old aluminum storm windows, they will try to run off with those to resell for scrap. Excellent money today in scrap aluminum, could be couple hundred dollars worth.
Also understand any contractor will give you about 15 minutes max per window. The homeowner can do a far better job. They also tend to measure the windows a bit on the loose side. When in doubt shoot more caulking. You will get no repair in the really nasty situations, that puppy is going in quick.
I don't like Lowes or HD, they have junk in terms of windows of all types. True of a lot of their gear.
The last windows I bought for replacement got thru a local hardware store (Part of the DIY chain). That is the other part of the problem. Finding a good source where the homeowner can buy direct without messing with one of those window companies. Can be done, save you a lot of money, not difficult to do. Some tricks in doing the measuring but it is not rocket science.