I never remember all of the fics that I think are amazing, so I have decided to keep a rec list I'll add to as I read. I'm not going to include all of the amazing fics I've read in the past because it would take days, maybe longer, to find and gather them all. With the exception of a small few I feel I'd be remiss not to reccommend, the rest will only be things I've read since this post was created so bear with me if it's a bit sparse for awhile.
First, let me rec the most amazing rec list I've ever seen. I go back to her lists all the time for new things to read and she's regularly adding to it.
Annie's Bookmarks has Harry Potter (all eras), Merlin, American Idol, etc. and I love her taste. I highly reccommend it. xD
Star Trek
Spirk (Spock & James Kirk)
Veritas by TheProblematique (186k words - NC-17)
Captain James T. Kirk and Lieutenant Commander Spock are hereby charged with negligence and wanton misconduct due to their emotional compromise with each other.
My Notes: It took a bit to get used to the writing style and I found the A/Ns to be distracting so I started just ignoring them entirely. There are some issues with some of the writing style throughout, but you get so engrossed in the story those get forgotten completely. It was very riveting and the story is very well told. The characters are highly believable and interesting.
Harry Potter
Drarry (Harry Potter & Draco Malfoy)
Owned by Jordan Grant (103 parts - NC-17)
The Potters have always owned the Malfoys. Post DH, epilogue ignored.
My Notes: Harry finds out he must take Draco as a slave to save his life and everything just gets more amazing from there. Jordan always keeps you guessing, never 100% positive what's going to happen next. She's a master of twists, manages to have fluff and angst in amazing counterpart, and truly this is one you'll be glued to your computer reading. Has a lot of BDSM which was a squick of mine until I read hers and wow. Don't pass this up just because of the warning.
Snarry (Harry Potter & Severus Snape)
If You Are Prepared by Cybele (Very long - NC-17)
Severus is asked to prepare Harry for the final battle.
My Notes: This is one of those that I mentioned that I just couldn't make a rec list without adding. I hated the thought of Snarry until I read this and I've been an avid fan since. This story is amazingly well-written, believable, and incredible. No HP slash lover should skip over this one.
*will be adding many more as time goes on*