Ranma best fits the Mercury quality.
His personality may be a bit hot headed, but in a fight, what are his qualities?
Speed, and Smarts.
He has a lot of power, which makes him Jupiter-esque, but he's beaten out in raw power by Ryoga there.
In terms of fighting, Ranma relies more on his raw speed, combined with knowing what and where to hit precisely. Then his HSH technique requires that he remain ICY cool.
Ranma may be boisterous, but his qualities are that of Mercury.