From my experience -
Polycom - Rolls Royce of the industry. Tempermental to configure, do not support STUN, but have a broad product line and a stellar reputation and loyal following (drink kool-aid......now). The best conference room phone around. They reportedly have simplified their firmware update mechanism...which is good news. Pricey, but if the customer wants and can afford, go for it.
Cisco - Cadillac of the industry. Name brand that everyone has heard. Not quite as broad a product line and more expensive than most. Good feature set, good looks, but in my market, the price is the big detriment.
Yealink - Chevrolet. Value leader with decent looks and a feature set that is, in many cases, equal to some of the Cadillacs and Rolls above. Easy to configure and supports uPnP in their latter firmware. Does not have the name brand recognition of the others, but price tends to to be determining factor. The t32 and t38 with their color screens is always a favorite with customers....at their price point.
Grandstream - Your father's Oldsmobile. A good value phone, but the product line appears dated and the manufacturing quality does not seem as solid, although I will say I have not had one fail. Some of the later phones show more modern looks and they just released a cordless DECT model (have not tried). Also supports uPnP. This one, in my opinion, gets more of the love/hate comments than the others. I still use them, but mostly the latter models - 1450.
Aastra - The Mecury. Big company which also handles the Meridian digital line. Kind of a luxury brand due to its digital phone system lineage, but does not seem to have the presence that the others enjoy. Still a very capable phone. Most have what I think is an overly large footprint, but they are very reliable. Some look a little dated, but the newer models are OK.
Panasonic - Buick or Ford. While not supported, I have used their cordless models quite a bit. There was some discussion that 3CX may end up supporting. Their analog offering has a huge following in the SMB market and I get asked about them from customers who have their analog systems or even cordless home, but thinking of taking the VoIP plunge. I know some on the board are using the wired models and I have heard nothing bad. I am not just not sure what price points they may cover; hence Buick or Ford moniker.
These are just my opinions, but I agree that there are so many factors that there is no one universal solution. I personally like the Yealink, Grandstream and Panasonic, but mostly because that is what my market can afford. They all essentially do the same thing, but perhaps not in the same manner.