XM: Betrayal by Valerie Jones (Gambit, various)

Valerie Jones is a Gambit-centric writer, both in her Betrayal universe and the Two Thieves universe (see prev. link) she shares with Lori McDonald (who is also fabulous), as well as in her X-Men/X-Files crossover -- all of which are simply stunning. I'll only talk about the Betrayal stories here because they were how I first discovered Valerie.

Betrayal has plot like there's no tomorrow -- time travel, paradox, action, romance, revelation, loss, sorrow, heroes and villains who possess deep reserves of complexity and humanity. It ought to be required reading for anyone, in any fandom. The only drawback to reading Valerie is that none of the series are finished (although there is plenty of material to read).

NOTE: Valerie has apparently evaporated from the web, and I've only been able to find a copy of the first story in the Betrayal arc at fanfiction.net. I'd appreciate pointers to the other stories.

ETA: As of January 2012, the third and last story in the Betrayal series is being updated here at FF.net.

L&O: various by Kitt (Jack/Claire)

In Law & Order, I think it's safe to say that Kitt is one of those writers whose name has been immortalized. She mainly writes Detective Mike Logan and EADA Ben Stone, and has two gloriously involved series devoted to the both of them. But I got into L&O because of a McCoy/Kincaid obsession, so Two Roads, Doors of Paper, and You Don't Know Jack are stories near and dear to my heart.

Kitt writes these two as gritty, troubled adults, who aren't necessarily good for each other, but still have had profound impacts on each other's lives.