Hi, Jane,
Ok, I confess, I have stayed in both the paradores in Leon and Santo Domingo de la Calzada. I am not aware that there is one in Burgos. In terms of "spectacular", the one in Leon wins hands down, but along with its spectacular architecture comes a pretty impersonal atmosphere, lots and lots of rooms (many of which are in a new wing, quite comfortable and modern, but not historic) and a so-so restaurant.
The one in Santo Domingo (in fact, there are two there, but I have only stayed in the one across from the cathedral) is very nice, not spectacular, but much smaller and more "cozy."
If you have the flexibility to go off the Camino for a day or two before starting your trip back along the camino, I would suggest that you take a look at Santo Estevo, a parador in a very old monastery in a remote part of the province of Ourense. I was there this past year and it was very very nice. And the surrounding area, known as the "Canyon of the Sil River" has some georgeous scenery.
The parador website is very user friendly, with a clickable map and a good description of each one.
Happy birthday, and buen camino!