I actually really liked Extras. I thought it was a very realistic description of how Japan would be in the future. It was very unique and exciting, too. ^.^
Specials I didn't enjoy, but that's probably because (spoilers in white) Tally turned out to be a cutter, then a special. I didn't like the way he described how she looked, and the kissing scene where she almost threw up bothered me. But I have to say I give Mr. Westerfield props for doing that to his main character. I think I just kind of felt sad after I finished Specials.
I'm going to say that Pretties was my favorite by far. I enjoyed reading about the way pretties lived, and (spoilers in white) I also loved how they figured out what was going on, and the scene where they both took the pills meant for one person. That was such a cool twist.