There's not a single character that I hate in this game, but if I have to rank them it will go like this:
8) Amarant: a fun character to use in battle and I enjoyed his relationship with Zidane.
7) Eiko: I find her more useful than Garnet in battle. She can be a little annoying, but she's just a six year old.
6) Garnet: my favorite FF heroine. She and Zidane make a really cute couple.
5) Freya: my favorite character to use in battle with a beautiful backstory and character theme.
4) Steiner: another comic relief and loved how he changed throughout the story.
3) Quina: no other FF character made me laugh out loud like Quina. I also love to use her in battle.
2) Vivi: not one of my favorite characters to use in battle, but his story pulled at my heart's strings. He's so cute that I want to stuff him inside my pocket and take him home with me <3
1) Zidane: of all FF characters, Zidane had the most impact on me and I just love his fun-loving personality. His line "In the end, it boils down to two simple choices: either you do or you don't" always rings in my head whenever I'm lost about something. He's also pretty badass in battle.
Favs: FF9, We <3 Katamari, Zelda: MM, Jet Grind Radio, Castlevania: SotN, Ape Escape, MGS, Chrono, Suikoden 1-3 http://myanimelist.net/profile/Haya_Yandere
Wait...there are people that say Steiner is the worst character? Seriously? He's easily the 2nd best character to have in battle (just behind Quina) and as a character he's flipping hilarious and awesome at the same time.
Ghaszaszh Nyirh. Of the Old Ones, the scrolls say that he loved the debasement of his slaves the best. And they go to him willingly.
If there were a metric to judge the characters by, this would be easier.
If I want to just murder through the game quickly, expect me to skip endgame Vivi and endgame summons. That's where characters like Zidane/Steiner/Amarant shine.
But If there was suddenly an extra boss. Something new and extremely challenging with six digit HP, my party would be different. For example, if you want to do the maximum amount of damage in the fewest turns, you would want Garnet/Vivi/Eiko and get them just on the edge of trance and start double nuking every turn for 19998. No one else can do that barring Steiner's 'Charge!', which is conditional. Not to mention, this would constantly keep you regening during spells/summons and hit the whole field.
But that kind of party isn't necessary or realistic except for scripted fights where you can manipulate trance beforehand.
Anyway, my catch-all, generic, go-to squad listing goes like this:
1. Amarant- For most satisfying Attack command and a VERY utilitarian Flair command. Also high speed.
2. Vivi- Spells do great damage throughout the entire game. Slap a booster on him and he hits like a TRAIN.
3. Eiko- White magic is always helpful. She gets Esuna, Phoenix, and Carbuncle.
4. Steiner- His physical attack is powerful, especially when tranced. Great defensive stats. Shock is an easy 9999 for those tough spots
5. Zidane- Physical attack is great. Flee for the "It's all over" moments and steal gets you some equipment earlier than you usually would have gotten it.
6. Freya- poor damage without Dragon's Crest or Cherry Blossom. Isn't very durable. Best thing about her is her Dragon abilities. She is a good substitute for Amarant in terms of utility.
7. Garnet- Nothing terrible about her really. She just plays like an inferior Eiko so she belongs down here. My favorite thing about her: she gets Berserk, for when you want to have a little fun.
8. Quina- Not durable, does variable damage, slow. Quina HAS to eat enemies to be worthwhile. His/her damage isn't great without gimmicks (frogs eaten/1 HP). Overall, without Blue Magic it is useless. With Blue Magic however, it becomes a perfectly viable party member with lots of utility and damage. But who has time to find and eat all the creatures? I usually ignore it mostly.