Irving.
In my admittedly naive assessment, Derrick Williams is Michael Beasley with a little less athleticism and none of the idiocy (which is pretty good). If the Jazz are planning on rolling with Favors, Millsap, and Jefferson, he might not be the best fit here if the Jazz wanted to see what would happen with him at the 4.
Perry Jones isn't a 3. He might be a perimeter-oriented 4 in the vein of what Lamar Odom has become and that's awesome, but he's also soft and doesn't rebound. Having what appears to be three PFs already making up the big rotation, it doesn't make a ton of sense to me. If Jones was a homerun, then you have to go BPA.
I like Harris, but Irving looks like a perfect PG. Not only jet-quick player (like so many of the PGs that have been taking over of late), but has a very well-rounded game, a jumpshot, and has good size for the NBA which never happens. Use Harris to try to get that mystery wing or something. But Irving looks transcendent and pairing him with Favors (and even Hayward?) is one hell of a young core moving forward.