There are a lot of great characters from quest mods that really had character and felt like they could've been in the vanilla game. They are well crafted by the hands of modders. These are my favorites:

Deadpan from Beyond Boulder Dome: This guy is just awesome. He is a pre war ghoul who loves cats, has a comical, yet well mannered sense of humor, and he has a sword with a venomous bite! Definitely one of the best characters in BBD.

Steven Randell from NVB1: This one is a OG. He was one of the first characters in a New Vegas mod that really got you invested in him. He had a great backstory, great personality, and a overall great bounty hunter.

Bobby Bass from New Vegas Killer: This is one of the funniest characters in the whole Someguy Series. He gets you gut busting. He only came to the Mojave Wasteland for two reasons, titties and biscuits in gravy.

Vincent Vincent from Vincent Vincent: Not only is this guy clever with his dialogue, he is very intelligent in his own way. He is that guy that everyone calls crazy, but the metaphors he tells us, the way he talks about stuff, he's a very intelligent dude. There are pinkies and thumbs in the wasteland, but this guy is the ring finger. He is very special and you just want to be with him forever. Can't wait to see him fully fleshed out in FO4!

Riddick from Dragbody's Riddick Companion mod: His character fits in so well into the Fallout world. He was a easteregg in Fallout:Tactics(right? don't remember). Great job Dragbody! Vin Diesel would be proud!

Bradley from The Inheritance: He is such a great character. He is very relatable is some ways. He has a great back story and a funny sense of humor. !!Spoiler!!

Spoiler:

Niner: He is that type of druggie friend you can't just help but love. You just see him as a brother. He is funny, relatable, and overall a very down to Earth type of guy.

Willow: She is by far my favorite companion and by far my favorite character in a quest mod. Her backstory is great, her dialogue is great, her voice acting is fantastic, and overall she is an essential mod for my playthroughs. I just love her so much. Great mod and is one of the best mods of all time for New Vegas.

I really like Niner. He's the perfect fit to round out my group of wasteland Expendables. I really like his character and his dialogue .

You basically listed all my favourite NPCs

Bradley was one of my favourites as well. Russell was a really good replacement though, he is one of my favourite modded NPCs as well as Niner and Willow.

Vincent, because he's so nutty and stupid that I can't help but love his goofy banter. Can't wait to see him in Fallout 4.

Randell was great, he was nice but honest

Bobby Bass... because he walked motherf***er. He walked.

Bobby Bass, he is one of the best and memorable NPC from entire game, I am waiting for follower version. Russel, Bradley, Vanessa and Bad Mothafucka are also my favorites.