Definitely Kankuro. Everyones all "Sasori" this, "Sasori" that! But what about his predecessor Kankuro, huh?! He's just as talented and has so much more to show, but unfortunately Kishimoto just gives him the shaft and doesn't seem to want to develop his character any further.
For those of you calling Sasori underrated, consider this: He has a bunch of hype behind him, he had a sob backstory, plus he has a strong fanbase to boot. That doesn't sound like an underrated character to me! But I guess I must have a different definition of the word "underrated" than most other people on this forum...