The Knight. Chaucer sets up the stall for the theme of class which is going to bubble throughout the Canterbury Tales:
Me thynketh it acordaunt to resoun
To telle yow al the condicioun
Of ech of hem, so as it semed me,
And whiche they weren, and of what degree,
And eek in what array that they were inne;
And at a knyght than wol I first bigynne.
Roughly translated, this quote might read as follows:
I think it's logical
To tell you how each person
Appeared and was, as it seemed to me,
And who they were, and of what class they were,
And also what clothes they wore
And therefore, with a Knight, I'll begin...
So right from the start, we're going to be judging by class, by clothes, and by appearance. And it makes sense to start with the person of the highest class (of course, the Host will also - likely - rig the story selection to let him go first) - the Knight.