I tend to like the majority of the characters and dislike very few of them. Admittedly I never played much of Final Fantasy 12 and I've never played Final Fantasy 13, Final Fantasy 3 or Final Fantasy 5 but I'm sure I'll be able list enough characters.
Favourites: Rydia (young), Edward Chris von Muir, Kain Highwind, Yang Fang Leiden, Cecil Harvey (Dark Knight), Relm Arrowny, Shadow, Yuffie Kisaragi, Cat Sith, Vivi Orunita, Eiko Carol, Adelbert Steiner, Quina Quen.
I added last names in order to make them more identifiable though some will need some additional explaining. I like Rydia (young) but found the concept of an older Rydia to be pointless, why couldn't they simply have her get stronger naturally. I also preferred Cecil when he was a Dark Knight, both his design and his stats. My favourite of all the characters is Vivi.
Least favourite: Rosa Joanna Farrell, Balthier.
Balthier I simply don't like but Rosa I found to be annoying as heck. Whenever she joins the party she is unneeded (I would use Rydia, Edward or Cecil to heal) and she is completely oblivious to Kain's feelings despite how obvious it is, showing great displays of affection in turn to Cecil.
Favourites: Rydia (young), Edward Chris von Muir, Kain Highwind, Yang Fang Leiden, Cecil Harvey (Dark Knight), Relm Arrowny, Shadow, Yuffie Kisaragi, Cat Sith, Vivi Orunita, Eiko Carol, Adelbert Steiner, Quina Quen.
I added last names in order to make them more identifiable though some will need some additional explaining. I like Rydia (young) but found the concept of an older Rydia to be pointless, why couldn't they simply have her get stronger naturally. I also preferred Cecil when he was a Dark Knight, both his design and his stats. My favourite of all the characters is Vivi.
Least favourite: Rosa Joanna Farrell, Balthier.
Balthier I simply don't like but Rosa I found to be annoying as heck. Whenever she joins the party she is unneeded (I would use Rydia, Edward or Cecil to heal) and she is completely oblivious to Kain's feelings despite how obvious it is, showing great displays of affection in turn to Cecil.