I'd say:
AJ (Season 1)
Jordan (Season 3)
Zach (Season 3)
and the most out of anyone:
Willow (Season 3). One of the sweetest people ever cast on the show, played such an intricate game, went on a journey in the house discovering things about her sexuality, I could go on. Unfortunately though, they barely showed her on the show so only feedsters really appreciate Willow. So I'd say she's by far the most underrated ever, as I could only name two people who even like her on this forum
Bless