Love the rankings, so I'm happy to contribute.
FF12 is my 4th favorite of all Final Fantasy's and also the one with the smallest cast. As such, it is INCREDIBLY difficult to rank my characters. I will do this as best I can, but it is quite a task.

1. Penello - While I found it difficult to not rank Balthier as #1 (so you can tell who is #2), I have to go with my heart. Penelo was my favorite character to use and my favorite character in the game. Despite the fact that she is probably the least important of all characters, and had the weakest back-story, there was something about her that did it for me. She certainly seemed wise beyond her years, and I liked the small romance she was developing with Vaan. Penelo was the kind of character I really felt should have had a greater role in the grand scheme of things, but FF12 was very deceptive in terms of who the story was really about. At the start of the game, I thought Vaan and Penelo would be focal points; but in the end they seem to mean the least. Penelo was a strong magic user and I always seemed to give her the Katana or Pole. There is a sentimental attachment I have with her that I can't quite explain, and that alone is why she's #1 for me.

2. Balthier - Balthier is the coolest of all characters in FF12. He's incredibly strong, incredibly savy, and super cool. He's what David Beckham would be if David Beckham were in a FF game. Balthier may have had some similarities to Han Solo, but for the most part was his own stand alone character. His back-story was awesome, his guidance of Vaan was awesome, and his love for Fran was deep. I never had a complaint with Balthier, and always enjoyed his dialogue. He was almost always in my group and a beast with the Zodiac Spear (I know, who wasn't...). Balthier was only narrowly edged out by Penelo.

3. Ashe - During my first play through of FF12, I was frustrated that the main character (Vaan) seemed to take a backseat to Ashe. Initially, I wrote Ashe off, but quickly realized she was an incredibly strong character. She was another whom I'd put the Katana on and giggle as she sliced the bejesus out of my foes. Her magic ability is off the charts and her Quickenings were badass to watch. During my second play through, I really bought into her story and her character. Ashe was another of my permanent part members who I always hated to pull out. There wasn't much I would have changed with Ashe, if anything as she just seemed to work out in every instance. She also had a cool look, cool name, and a bitchen outfit. She personified the female protagonist and felt like an evolution of Celes.

4. Fran - There are no losers on this list, as I really cared about all the characters. Fran was another character whom I didn't like on my first play through, and then fell in love with on my most recent. She was complex, powerful and had a great back-story. She sort of fell from the "big picture" towards the end, but I never stopped enjoying my runs with her. She fits into my gripe with FF12, in that I could never get as much play out of my favorites as possible. If it would have been 4 person teams, I would be thrilled; but alas it was only 3. I would have enjoyed a little more detail into Fran/Balthier relationship; but I would take what I could. Fran with a sword/axe was my favorite combination. She is sexy as hell also. Seriously, who didn't love the Vera?

5. Vaan - Some people hate Vaan, I am most certainly not one of them. I always struggled with determining his true age, but the character was fun for me. The fact that characters could develop as you saw fit was a huge selling point for this game. Vaan gave me 100% flexibility and I relished that. The "bait and switch" of him being the main character through Act 1 and then taking a back-seat to Ashe was a little troubling at first. But once I accepted Ashe as awesome, I was cool with it. Like Penelo, I felt Vaan really stopped mattering in the story arc and wish their relationship could have been expanded on. I wouldn't have minded a little bit more of SOMETHING, but I'm not sure what. I thought Vaan's entire beginning was a little weak; and I just never cared for the Slums of Rabanaste very much. He did develop quite well and ended up being strong enough to keep me interested.

6. Basche - As I said with Fran, no losers on this list as I truly loved all the characters. Basche was my favorite during play through #1, but my most recent playthrough had me using Basche less and less. Now, that isn't to say I dislike this guy at all; it's just he did the least for me emotionally or personally. He was a very strong character with a very strong story; and he mattered through the entirety of the game. He was an absolute monster and felt like the main character through portions of Act 2. There isn't anything I can pinpoint that I would do differently with Basche. He was fully developed and fully realized; but did not resonate with me as much. I think he suffers only because I didn't use him as much as I did some other characters, and for that reason alone; he ranks a little low.

I would like to note that I do love all characters from Final Fantasy 12. If not for some nostalgia of FF4, it would sit comfortably at the #3 spot. It also represents my most painful game, as I recently put 80 hours in coming close to all ultimate weapons; and my PS2 deleted my save. So, f this game