I have all three of their most recent Boeing products and I enjoy all three of them.
737: I have to say of the three my least favorite is the 737 as it is the original of the three and slightly less detailed than the 777 and 747 in subtle ways and has a few small issues such as the ailerons being nautally tilted down slightly or the rudder being too responsive especially when decrabbing. The engines start slower than the real thing too. It is also the least customizable. The sound pack it comes with is is pretty good, but again compared to their other products, it is the least realistic. Some of the lack of features however is the 737's fault itself as it is quite primative compared to the 777 and 747. With that said it is quite easy to use and I enjoy it very much despite not caring for the 737 much as a plane in real.
777: This is the second of their products and it is noticeably newer than the 737 in a few ways. It is about as customizable as a 777 would be in real and they way they were able to replicate the systems and their functions is quite something. There is very little I could find fault with on this one. I have to say it is the easiest of the three products to use because the 777 is so much newer and more advanced of a plane in real. It has the built in checklists which are also very helpful. If you start the cockpit from a turnaround state, the setup is very quick and simple. The sound pack it comes with is quite realistic featuring the various sounds the 777 and the GE90 make inside and outside the cockpit. The only thing I wish I could have is the 777 "classic" and it's lack of raked wingtips and different engines because then if I wanted to fly a United 777-200ER for example I wouldn't have to use the fictional United 777-200LR. That's not a big deal though. It is a very stable and polished product and one I absolutely love using.
747: This is their newest product of the three and is very similar to the 777. It is somewhat even more customizable partly due to it being a newer product, but also because the 747 is. It features various different configurations and all three engine types used on the -400. The engines all sound different and very accurate and all have their own characteristics inside an outside the cockpit. The ground handling of this product is where they made a significant advance over the 777 and 737. It comes complete with various service vehicles and cargo loaders and air stairs. One other major difference is that unlike the 777 and 737 where the ground connections are instant on and off, on the 747 you have to request them and wait for them to connect. Despite having a level of automation between that of the 777 and 737, the 747 can be a bit confusing to use at first, at least after using the 777 and 737, as some systems can require a lot of switching and mode changing such as the fuel system which can get a bit confusing with multiple crossfeeds and override buttons. The sounds as I already mentioned are again very realistic for the most part and I appreciate on this on the buzzing of the fan blades on the engines fade away as you climb just as in real. The level of detail on it is so high that they have simulated even things as insignificant as the visual difference between equipping the #2 and #3 hydraulic systems with air driven or electric motor pumps as the air driven ones have a small exhaust grille on the engine pylon where the electric ones delete that. Another issue that I understand is due to they way they have set up the sound system is that all the engines sound like a GE90 starting up. It is an extremely stable aircraft other than when it sometimes freaks out in turbulence and shakes like crazy, but that might be a glitch and hey may have fixed it. There are a few other nitpicky things I can think of such as the aux hydraulic pumps not having a sound when they run or the fuel pump sound file not playing when the APU system automatically turns them on, but these may also have been fixed. There is a possibility that they may be making a 747-8 expansion at some point.
All in all, any of these PMDG products do not disappoint. One thing I wish they could add to all of them are features such as openable windows and a few other similar miscellaneous things you would find in a cockpit. I also wish that they would make more products such as a 757 and 767, but that many would be unrealistic considering how much work they put into a product. The biggest downside is the price, but the product certainly justifies it. If you are like me and like to operate real flights only with the correct type of planes, the 777 will take you much farther, if you don't care so much about that than the 747 is just as convincing. I think the 777 is my favorite by a hair as there is something about watching it's heavily computerized systems do what they do I find fascinating like the automatic initialization tests done when first powered up. I suggest you watch some videos on how they function before you make up your mind, something I can't really do which is why I have both.