My pick: Toshio Furukawa
Reasoning: I love how Toshio Furukawa can sound snarky and arrogant one minute and really serious and sharp the next, depending on Piccolo's mood. He's got that devilish feel to him. However, it's not a "character voice"; it sounds natural in every scene. As far as the English dub(s) are concerned, Scott McNeil's Piccolo edges out the competition by having a wild flare to it, while still remaining solidly rooted. In comparison, Sabat's Piccolo seems a little bland and the others just sound plain bad.
Breakdown: Toshio Furukawa > Scott McNeil > Chris Sabat > Unknown #1 = Unknown #2 = Unknown #3