Erik, you've asked the right questions. Each time one of these "What is the best scanner" threads opens, there are as many replies as there are models of scanners - all claiming to be the very best you can get. Measured against what?...is the correct question. At least to my way of thinking, if you don't have a lab with sophisticated equipment to use, all you can try to do is measure performance by ear with radios side-by-side connected to the same antenna at the same time.
I've done those tests many times with just about every scanner on the market capable of receiving the air and MilAir bands in AM mode (118-144, 148-150.8, 225-399.975). I use the same antenna through an active multicoupler. For radios I don't own, I've invited friends to bring theirs and hook them up here. Each time we do those tests, one radio stands out over all others.
As far as I'm concerned, the Uniden 785 is at the very top of the pile. I'm speaking of scanners now, not general receivers like some of the Icoms I own which I don't consider to be scanners. For instance, my Icom R8500 will outperform anything I've ever used but it's simply not a scanner. And for the price, it damn well better perform better than a scanner!
For a very long time, the Uniden 780 got the reputation as the best for MilAir in particular and still seems to enjoy that reputation. But, at least in my case, the 785s outperform it every day since I use both here on a constant basis. I live in an urban area with lots of high powered transmitters saturating the spectrum so I need something that will not suffer desensing and interference from all those transmitters. My 780s get trashed by some of those signals that don't bother the 785s at all. Other scanners I own, I only use for specific purposes. For instance, the Radio Shack 2042 is a fair radio but I can only use it for monitoring military Command Posts within a reasonable distance of my location. It's not nearly sensitive enough for distant, weaker signals by comparison to several other radios in my arsenal.
Radio Shack radios in particular, are not very good for monitoring MilAir simply because most of them are not capable of the UHF 225-399.975 freqs or they won't switch to AM mode on the very important 138-144 mHz band. I believe I've had every Radio Shack scanner capable of monitoring the MilAir bands hooked up here on my antennas and they simply do not perform nearly as well as Uniden 780 or 785 scanners. I also own some older Radio Shack radios such as my 2005 that was a great scanner in its day and had great audio but it lacks sensitivity compared to today's scanners. It's still good for police/fire monitoring but lousy for MilAir. That goes for the 2004/2006 models too.
My bottom line would be that if you wanted just one scanner that's both sensitive enough to hear distant signals and not prone to getting wiped out by interference, it would be the Uniden 785 -- followed by the 780. If you use your scanner for trunked systems, the 785 can do that too in both analog and digital modes. However, once the fabled "rebanding" happens, the 785 will be useless for trunked public safety signals because Uniden will not support the changes that will need to be made. My reply to that is that I simply don't care. I use the 785s for MilAir listening and they'll always (well, at least until something better comes along) be the best thing out there for that purpose.
Yes, other makes and models can be used for listening to aircraft but in limited situations. You'll swear they are doing a great job - until you put it next to something that performs better. There are all kinds of exceptions for individual situations. For instance, if you live just a few miles from an airport you want to monitor, almost any scanner capable of the correct frequency range and mode will suffice. If you're interested in listening to distant signals, if you live in a noisy environment, if you want the capability of sufficient numbers of channels in your scanner, if things like a good display with alpha tags and easy to use buttons and knobs are important to you, get yourself a Uniden 785, or several of them.
The above thoughts are simply the result of monitoring MilAir for many, many years and the impressions I've formed while doing so. In the grand scheme of things, they are worth what you paid for them....zip.