For me, it has to be Klar. He's in his second incarnation now (his old form was vinyl) and he's been through four different wigs, two pairs of eyes, and two lovely face-ups by Kiki. On top of that, his current form is a hybrid between two companies, one of which doesn't sell the bodies separately, and he owns the most expensive outfit out of the whole group.
He was also the reason why I brought Aurym back as a character, who then caused me to bring Viatti back, too. Such a horrible chain reaction from a doll whose character I didn't even make before buying him! XD
And, no. I'm not going to try and calculate how much he cost me because I like to pretend that he wasn't all that expensive.