Among the 3, Taiga showed most improvement and she is barely recognizable now from episode 1. In terms of character development, she wins by huge mileage. She might still have the violent streak but it is tempered with compassion now since she is receiving human warmth that is markedly absent in her entire life pre-anime series. She is perhaps one of the loneliest girl in anime series I seen so far but thanks to certain someone, she is now getting back on track on being a self actualizing person. Which is very impressive to me.
Ami's refusal to be honest is the reason why she is stuck with her own impasse, hence her frustration is bottled up and she often take it out on Taiga. Worst part, she knew it herself. Her character development is glacial but explosive when it happens. A worst manifestation of her Jekyll/Hyde personae dramatis. She is hot, seductive but ultimately stagnant due to her own mind games she played on herself and Ryuji. However she believes in action and decisiveness, which is why realized from day 1 that she missed the bus sorely in fight for Ryuji's heart. She settled to be a counselor and devil's advocate in the series but her emotional pain did peek out once in a while in tantalizing moment which predictably missed by Ryuji not just because he is stupid but also he has reason to believe she is faking it due to her record past. Just as Taiga, she is another very lonely soul but she did get her kickbacks a little in this aspect but unfortunately she is too conceited to admit it to Ryuji especially. This is why despite her fascinating complex character, she is "stuck" now.
Minorin fake cheerful self is counterproductive despite her inherent kindness, she is seeing a future beyond relationships despite her feelings for Ryuji. Her semi ignorance of his feeling after the Christmas episode is major negative point to me and I actually agrees with Ami's anger towards her. It looks like she is trying wriggle her way out from this by do nothing. This is worst thing a person can do in my opinion. Minorin is another lonely girl but she covering it up with her antics with propensity to run away from any emotional conflicts if she sees fit. Or do nothing. She is not mature and selfish, certainly leaving people on the lurch but quick to anger if these being pointed out. On good side though, she is a loyal friend.
This leads to serious emotional catfight between a stuck and a half truth while Taiga is stumbling towards Ryuji in steady pace at expense of other 2.
Edit: (something from 4chan, an interesting thought on Ryuji)
Ryuuji isn't very well liked because he looks angry all the time. His denseness is his defense mechanism against his off-putting looks, but he isn't a retard. It's just his defense mechanism kicking in, because he's always been wondering if there were going to be people ever liking him.
He's a very nice guy and he's very perceptive of other people's needs, too. However he can't reciprocate or even understand Ami's suggestions or even Taiga's actions because he cannot simply understand that people like them could ever like them romantically. I think it's very realistic; I mean, if I befriended a hot model and she'll insinuate that she'd like me I'll think it's just all bullshit and I'll just be hurt eventually. That's what Ryuuji thinks about Ami, and that's why she could never get her emotions through. In addition to that, he hasn't realized Taiga realized her emotions for him because he simply plays his role as a father.
Taiga as a girlfriend to him, while subconsciously desired, isn't really one of his visible options: to him, he's a father and a guiding hand even if subconsciously the imaginary house they built among themselves is toppling down. This was why Ami was angry toward Minori: things can't always stay the same, and that's why she tries her best to reveal the truth of everyone involved. She herself has invested her feelings that she couldn't see it end as it had begun.
