I've used all 3.
NPVR (formerly GBPVR) is the first DVR I used. Allows you to use PCs as extenders.
I decided to switch to Windows Media Center a few years ago because I was having many issues with it. I usually mess around with it whenever a new version comes out but have never been able to get it to work without issues. I do have to say that the author offers great support and their forums are very helpful.
Windows Media Center is the easiest to setup and has a great interface. The issue with it is that you can't use other pc's as true extenders. In other words no Unified Guide. No stop in one room and resume in another. You either have to get an XBOX 360, Echo, or some discontinued used extenders on ebay.
MediaPortal (what i'm currently using) has a bit of a learning curve. But once you have it setup It's very stable (at least for me). Even more stable than WMC. I used to be afraid to Windows Update my systems for fear of it breaking something. (which happened to me a couple of times. My experience, yours may differ). Especially the new 1.3.0 Beta version. The new Titan skin is great. Although I still miss the quick categories section in WMC's guide. The reason I switched to MediaPortal is that you can use another pc as an extender. So I can stop watching something in one room and resume in another.