Could you tell me currently who makes a good AR rifle (and the specific model)?
Almost any of the major brands mentioned above will be good, reliable ARs that more than meet the criteria you have outlined for reliability and accuracy. However, all manufacturers occasionally produce lemons and this is true with ARs as well. I have found Colt to be the most consistent in good quality ARs; but my customer service experience with them has been horrid. Bushmaster is currently my preferred AR brand. They do tend to have more minor issues (excessive windage required to sight in, purplish hue to parts) than Colt; but their customer service is good about fixing them and they rarely have major issues.
Having said that, I wouldn't turn my nose up at Rock River, DPMS, or Armalite either if the price was right. There are also the kit houses like JT, Model 1, etc. They can be good if you have a limited budget and don't do a lot of heavy shooting; but if you do a lot of shooting budget an extra $100-$150 to replace or fix things that go wrong on it.
What is the price? My experience is that they usually cost more than other rifles, why?
For one of the known quality brands mentioned above, plan on about $700-800. If you are willing to take chances on used parts or parts kits and build your own, you can get them down into the $550-650 range; but it isn't uncommon to see issues with those rifles either. I'd do some research before I went that route to minimize future problems.
Is adding a scope to the AR rifle easy?
If you buy the flattop version it is.
If I wanted to put optics on an A2 configured model, what's the process? Is it recommended and/or possible?
Well, you will be limited in your choice of optics. You will either need to buy one of the scopes specifically designed for the A2 carry handle or you will have to buy a rail to mount in the handle and then mount the scope to that. This may cause issues with being too high over the bore to get a good zero and also causes poor stockweld for many people. A third option is to buy a bilevel rail that mounts the optics at the proper height but out over the handguards - this only works with optics that have unlimited eye relief though (basically red dot/reflex type sights).
In addition, any time the scope is removed from the handle (and that could be a lot since an AR15 with carry handle already is pretty thick in that dimension and barely fits into many cases and racks - not to mention cleaning), you will need to rezero it.
I've owned an A2 and mounted an ACOG (designed for the A2 carry handle) in it and even with a very-high end scope built especially for that purpose, I still found having a flattop much more convenient.