Ansem wasn't around for a long time, either; granted, there's obviously a big difference between how long the two of them have kept hold over their respective vessels, but I think the important thing is that they had their place in the flow of time from which YX could yank them out of.
My current list so far:
Terra-Xehanort (Present, in the form of MX)
Terra-Xehanort (past)
Young Xehanort
Riku-Ansem/Ansem SoD
Young Riku (past, after leaving DI)
Terra (past, when he still liked MX)
Xemnas
Saix
Xigbar
Xaldin
Vexen
AtW/DiZ
Vanitas
Young Riku, Terra, and AtW are probably the ones that would beget the most questioning. I think the most important quality for a vessel to have is a shared desire with MX. In that sense, MX would use desire as a bridge into someone's heart. Both Riku and Terra desired power, and AtW desired understanding, all three examples particularly in relation to darkness, hence the term, "Seekers of Darkness."
Young Riku and Terra are characters susceptible to darkness, and therefore susceptible to MX; this might be primarily speculative, but I don't think either Riku or Terra can be swayed by a five-minute history lesson in the present in the same way that they can't change their pasts as vulnerable and naive people.