Most fulfilling? I'd say my Courier in Fallout: New Vegas. She looked great, and she had an interesting outlook on life (personal, for instance). "A naive lesbian with a heart of gold", is probably the best way to describe her. Despite being of pure heart, diplomatic and never killing people/creatures unnecessarily/unjustly, her two favourite pasttimes are gambling and sex.
The most fulfilling part, was when she killed Cook-Cook (one of the 3 raiders with a bounty on their head). The only time she ever
wanted to kill another being. He had raped and burnt a woman named Pretty Sarah, giving her 1st or 2nd degree burns all over her body (ironically, she is no longer really very pretty), and raping Corporal Betsy (from theat sniper unit) who ended up having a mental breakdown and hitting on every woman she comes across.
The main reason it was so fulfilling, is the way I killed Cook-Cook. In VATS, I aimed at his incinerator to break it. He saw me as I tried to aim, and I shot straight down the barrel of the weapon. Amazingly, Miss Fortune came in, and she sent him flying into the air - he literally went up in flames as his incinerator backfired. He flew in an arch shape, I realised at that moment, I wouldn't get another shot, so I decided to kill him in mid-air. I was using a rifle, and I didn't have enough AP to fire another round, so I exited VATS and managed to kill him before he landed on the ground (the line by Agnes Skinner from The Simpsons "They'll kill you 5 times before you hit the ground!" comes to mind).
It was so fulfilling, because it was as if the game itself, sensed my passion for roleplaying my Courier, that it decided to make sure she killed him in the most glorious of ways!
Another fulfilling moment, was when my Courier managed to understand Christine Royce, despite the fact that she had no voice box. She figured out that Christine was talking about the Brotherhood of Steel and her girlfriend, Verconica Stantangelo. I almost cried when Christine (with Vera Keys' voice box) told her that she would endeavour to protect the Sierra Madre, instead of joining her into the Wasteland (I left Veronica and Rex when I started the Sierra Madre add-on).
My Courier wanted to reunite her with Veronica, though she understood everything about why Christine decided to stay (what with their similar good hearts and beliefs). I like to think she bawled her eyes out Christine told her.
There was also the part where she had to choose between saving the Mojave and letting the Lonesome Road ED-E "live".
I also loved running around looking like some sort of super spy from the not too distant future, thanks to the texture problem with the female version of the Grimy Pre-War Businesswear which looks incredibly hot (much more than the correct texture)!
I've given her a somewhat huge backstory. Long and at one point, kinda lame, TBH. From off the top of my head...
Warning, this story might be a little graphic or PG-13 (

) for some or a lot of you.
My Courier's mother and father both lived and were born in New Reno (I'm a huge fan of Fallout 2, by the way). The story is from the perspective of the Courier and her mother. Her mother name is Vivienne, and she was a prostitute working at Miss Kitty's place, the Cat's Paw. She never met the Chosen One. Vivienne had short light-peach coloured hair. She started working there when she was 19, in 2261.
After a good few months there (as far as good goes), a new customer came. He had already been with all the other prostitutes when they first "met". He had dark red hair slicked back. He was a new made man of the Bishop family (currently owned by Angela Bishop's son, at the age of 18. Angela Bishop is the daughter of John and Leslie Anne Bishop and is the mother according to my story, and as some would have guessed, the Chosen One is the father). He was 20 years old.
He started to frequented her services, and thus became her best customer. She was obviously his favourite. Though she never knew for sure, Vivienne thought he was interested in her. She became attached to him and indeed, fell in love.
One day, several months later (here comes the lamish part), he felt protection wasn't enough, and wanted the "real thing". Being the nice person she was, Vivienne didn't want to break Miss Kitty's policy. Being the confident and upstart young man that he was, he felt he didn't need it. He told her that he didn't want to use it, but she said she'd rather do it after her shift, instead. He was too impatient and just went for it. He forced himself on her, despite her resisting. Eventually she gave up.
Miss Kitty heard the commotion and sent for the Shark Club's guards, as she has steady contracts with the crime families such as the Bishops. The bouncers came in and took him away. As you can imgaine, Vivienne was very upset, but she was upset because she didn't want them to take him away. Unfortunately for her, that was the last she ever saw of him.
9 months later, in the year 2262, she gave birth to her daughter, whom she named "Vinaiya" (I got the name from my Dalish Elf in Origins). As Vinaiya grew, Vivenne took care of her, while still working for Miss Kitty. When she had to work, the other girls would take care of the would-be Courier. She never really had anything to do with Kitty's Hawker outside, so as she grew, she spent the majority of her time around the women (hence her orientation as a lesbian).
One day, shortly after her 14th birthday, her mother lost all hope of ever seeing the man she loved again, and introducing him to their beautiful girl. She killed herself. The young girl cried for a week, after giving her mother a brief burial, and made herself sick. She gave her mother a brief burial that day, however It made it hard to draw in customers. A month later she managed to to feel better.
3 years later, she decided that she would leave to have her own life. Miss Kitty offered to let her stay as a prostitute, but she declined, saying that most of the customers were men, and that she would rather stick to sleeping with only women.
It didn't take Vinaiya long before she began her career as a courier. Several weeks later, her job took her back to her hometown, though she wouldn't stay there for very long. Eventually, at the age of 19 she ended up in the Mojave, before being captured by the Khans and getting short in the head by benny.
So yeah, pretty fulfilling roleplay experience there. Long post is long, BTW.