Out of those three by Neal Stephenson, I would say absolutely read Cryptonomicon first, and probably then the Baroque Trilogy. Seveneves I found to be his weakest book by far (I've read all his novels except for Anathem and his very early work), and Reamde I thought was the second weakest (not actively bad like Seveneves was in places, just not as good as his other ones - I've described it in the past as the world's best airport thriller).
I've only read one of those three by Guy Gavriel Kay, so I can't advise on which one to read, unfortunately.
I've only read one of those three by Guy Gavriel Kay, so I can't advise on which one to read, unfortunately.