I think the Via de la Plata is my favourite, although the Francés has a special place in my heart, the routes in France are very beautiful, the Norte, the Portuguese and the Madrid have their charms! Like my grandchildren, not a bad one among them imo.
The greatest challenge of the Plata? Its length. It is a really long walk. Quite a few of those who started with me in Seville did not stay the course because of injury. But for me that was one of the things I liked the most. The long relatively flat stretches at the start, meant that I was pretty fit by the time I reached the mountains (I took the Sanabrés). Many times I felt I was flying, when the earth rolled past and my spirits soared. The length and the rather "bare bones" nature of the Plata I think contributes to forming bonds with other walkers. Over such a long walk people came and went, groups formed and dissolved, but I made several lasting friendships - and have since walked other paths with some of those friends.
It also felt so essentially Spanish, the real heart of the country. Culturally of course there are some outstanding cities.
I went in early spring, and I think that is the ideal time, walking with the season. The wildflowers were extraordinary.