Hello there,
title says it all but let me explain a bit.
I always find myself developing a wide range of concepts before a new saga begins and more often than not the concepts that seems promising on paper aren't the most rewarding. So if you could share your experience with character that were actually played and enjoyed and explain why those characters were satisfying I would be glad.
The last one? 
Truthfully, I find I get the most enjoyment out of a character who can do something well out of the gate, and not who is designed to be able to do something well in the future, if his advancement plans end up panning out if everything works perfectly. This goes to the idea of playing a specialist, and then trying to use the specialist to solve all problems using their specialty. If I can do something well, with a character out of the gate, I'm less concerned when the advancement plan goes out of the window. The first example I am using below has no particular advancement plan. The second example was determined to pursue Corpus, but had to take an apprentice earlier than he planned (he's 13 years post gauntlet) because the House needed it..
I've had great fun with my weather maga who successfully used The Incantation of Lightning to prove the villany of a maga of the Order to a crown prince. Before she spoke, she said that may lightning strike her should she lie. The mag lied, so lightning struck her. No words, no gestures, and a finesse roll to make it appear that the lightning came from someplace other than my maga's hand, and I proved the lie to the satisfaction of the prince. The maga never died, because it was cast without penetration... Now, she can't do very much else except weather magic well, she has some facility with corpus, but that was an effort to get to a good Longevity Ritual. She has gigantic holes in her Arts, can't open an apprentice's Arts without creating 6 or 7 deficiencies, and is 22 years post gauntlet.
I have a Corpus magus in another saga, who is almost asexual, he's very cerebral and an academic and is in House Mercere, which has a clear duty for all magi to reproduce to continue the Magical Mercere line. He was pursued by the daughter of a count, who's married. He has envied beauty, and her husband learned of his wife's infatuation which led to attempts on the character's life. Meanwhile the randy daughter of the count was relentless. Finally, my magus had to cast a spell that would satisfy her, because he wasn't going to do it (he'd satisfied his duty to the House and had started the LR, so what's the point?). What would a man who isn't really interested in sex do? In a short-sighted manner, he chose to cast the spell, because it would get her to leave him be. Of course, he botched the spell and he satisfied her so well that everyone in the manor heard her, including his apprentice, another count and his retinue, a covenant companion, and several of the grogs. Now every woman present will want what the count's daughter had and the men will all hate him for it.
Edit: homophones, Oxford commas.
Your last character seems to be quite funny indeed ! It must be a challenging role-play in the long run.
I always revolve around specialists lately too, for the same reason you do and because this way with the huge gaps in what the character can do, I'm sure the less experienced player at the table still have their moments of glory. That's something which happened to be quite important because I used to play in a role playing club sort so there was always new players.
I rather enjoyed playing a creepy prophetess in a recent short lived game. She had divination out of the gate and had mastered a spell that changed her into a raven. While most of the covenant magi were all clubbing in together and working hard, she would scare the locals, act mysteriously and generally creep people out. Of course when she did decide to muck in, in turns out divination is extremely powerful so the other magi granted her the eccentricities.
Also rather enjoyed playing the apprentice to one of my other characters. His master was a necromancer, creep and general nasty piece of work, while he was bookish, pacifist, obsessed with mathematics and terribly ashamed of his masters behaviour. Was great fun playing someone who absolutely would not engage in deeply immoral behaviour after playing the master who rarely did anything but.
So divination was strong in your saga ? that's something I've never seen actually played.
My favourite character I ever played in Ars was Itea ex Thessaly o House Ex Miscellanea.
She and her twin sister were both born with the gift and they were very close to each other - to the degree that their gift is intwined. Both were taken from their parents by the Order and without knowing the link between them, they were seperated at the start of apprenticeship. She ended up in the hands of a Witch of Thessaly who belonged to the order and her sister became a Guernicus.
What is fun with playing her is that she is a warm and loving woman who loves all the covenfolk and happily spends seasons with them or making them minor magical items, but that she has the personality trait that if she feels threatened or if anyone wants to learn her secrets (Summoning etc) she will vote to kill them most horrible. Add to the fact that she is a necromancer and has a ghost wich serves as guardian for her lab. Most magi of the covenant (except her Guernicus sister, who is also a player character) see her as a borderline infernalist as she is a devout follower of Hecate and fear her.
I started playing with only necromantic spells that affect ghosts, but as the campaign progressed, she became a healer and a very good one at that. But as she infused all her spells with her Chtonic Magic, most players in that campign rather let themselves heal slowly than let her heal them with a blessing to hecate (with the appropiate animal sacrifice) 
It's interesting that many of you like necromantic characters, in the few IRL saga I was part of, there was always some kind of necromancer, with different twists and most of them were secondary healer. This might be something to consider. Healing using chtonic magic seems like an interesting roleplay opportunity. Thanks for sharing.
I did not really understood how the link between the sisters came into play, could you develop that ?
Ah yes, forgot about that: Both characters selected the virtue "Special Circumstance" with a trigger activated it when we are within 10 paces from eachother.
I liked the concept of a Necromancer that actually is not evil, it was just something that she was taught by her master (who were a bt dark and omnious) and very secretive. So Itea grew up learning things that her master thought would help the Witches and herself.
So the Chtonic magic made her a really good necromancer when the Chtonic bonus and the Magical Focus both were applied and a very good healer and creator of LLRs. tThe fun thing about her was that she made her lab in a ruin of a temple to Hecate, and she had a latent magical ability which appeared later to be that she sometimes can talk with her Goddess, which made the Goddess answer when she had time and punish her if she bothered the Goddess to much.
My very favorite personal rpg character of all time is Roberto of Flambeau. I brag about him a lot. He is online here on the forums. Now, to explain why he is my favorite, and why I feel he is successful, can possibly be wise advice for others to consider when designing a new character for this or other games.
Mainly, I designed the character to be played. To be proactive, go out and do stuff. As a magus, he is specialized in doing stuff. A lot of spell mastery, Flawless Magic enhanced by an accumulation of a wide variety of spells. Highly specialized in signature spells. For the initial design, that took up only a little potential. Other traits also focused on doing stuff, More starting xp, self confident, and so on. Even his Flaws. Being both Overconfident and Reckless means there is little reason for him not to do stuff.
What has kept him fun to play? There is always more stuff to do. Being good at doing stuff, he learned how to do a lot of different stuff right away. Somewhat specialized in some stuff, there was room for improvement in other stuff. Then the need to keep the edge on the special stuff. And he keeps doing stuff. Leading an active life leads to a life of activity.
Another thing that makes him fun to play is his personality. Personality goes a long way. A lot of my characters have similar personality traits. Not all, just the ones I personally play over an extended period. But for this guy, the traits fit, and other aspects are more fleshed out, My interest in the character, and the fun I had playing him, inspired me to flesh out his history and personality more and more.
So words of advice to be summarized...
Roberto is actually already stored on my computer as well as your take one the "apromor" Flambeau. I find them very interesting even though they don't allway fit my playstyle, they're a bit archetypal maybe but I use them to remind me of what's a Flambeau like, to set up a standard when I decide to make twist while designing a character from this House. I undertand better why you focused on such personality trait that I sometimes tought a bit too "easy" when reading the character sheet without your explaination.
I think Jonathan.link nailed your first point, already but that's still interesting to hear you share his view (and mine) on the matter.
Point 3 is also interesting, I always blame myself for betraying the personnality I chose at character creation, maybe focusing on only a few well defined characteristics that differ from my personnality will make it more consistant.
Thanks for your imput.