Default DG is an indestructible physics-defying demonstration intended to serve as a tutorial craft for new players, default craft for the demo scenarios, and to help modders understand how the system works.
XR-1 is part of the excellent Altea Aerospace XR series. It uses the same model as the default DG but with fairly realistically modeled damage simulation and more complex and challenging flight controls, with optional modifications to make it more or less realistic. It's a great spacecraft, though I strongly recommend the XR-2 Ravenstar instead, as it's even more lovingly crafted with even better features.
DGIII and DGIV are part of Dan's series of addons, with the DGIV being a direct sucessor to the DGIII. In a very similar vein to the XR-1, it features realistic damage simulation (with actual destruction, so you at least get to watch it burst into debris and become a pretty fireball when you mess up), life support, options to make it more or less realistic to fly, and so on, with an excellent UI module for setting it up in the scenario editor. It's a lot of fun to fly, and I really recommend it alongside the XR-2 if you want to have a challenging futuristic spacecraft to play around with.
Also if you like big spacecraft, try out the Delta Glider EX. Haven't used it myself, but it's apparently one of the more popular addons, at least as of the last time I checked Orbit Hanger.
