1: 1. Jessica by y4y Getting it on with an enthusiastic 12-year-old is common enough as a story. Here I see a sweet and (relatively) believable progression from a casual encounter in the park to intimacy deep in the woods. Keeping it believable, he doesn't take her virginity (which in so many stories hurts for few seconds before leading to unbounded enthusiasm and orgasms -- yeah, right).
2: Sperming My Nieces by Dr Scribble I like lots of Dr Scribble's stroke stories. They're enough tongue-in-cheek to give a smile to the basic implausibility (as of most good stroke stories), but not too much. Here there's a prohibition that makes it exciting, since he can't slide his cock back and forth repeatedly in a niece's pussy. There's fun at the "edges" this forms to separate it from ordinary sex.
3: A Storm Come True by Tara The 2nd part in particular is a great stroke story, to my taste (including just-pubescent girl and incest), and being told from the (enthusiastic) girl's point of view helps a lot. I like several of her other stories too; would be nice to tell Tara how much I liked it, but she doesn't accept emails.
4: A Very Lucky Man by Vidi Veni Not an unusual fantasy, maybe, to have ongoing sex with your nubile stepdaughter -- with her and her mother's enthusiastic consent. This story is about as believable as this kind of scenario can get, and the sex descriptions are detailed, evocative and flow well, without getting crude and frantic.