Here be stories that have a combination of genres, or are hard to classify.
This is so perfect it hurts; I requested this pairing in the rose_lives drabble tag thinking you could do interesting things with this combination of characters, but had no idea something this amazing could result. Rose and Jack fight the Time War with the Eighth Doctor, and it’s dark and sad and time twists on itself in the most glorious ways (while remaining truthful and plausible). Gorgeous writing and characterizations are the icing on the cake, and the result is a tiny piece that nonetheless evokes an entire, fascinating universe.
While no longer canonically possible due to the end of S2, this remains a terrific look at Sarah Jane and Rose after Rose's adventure with the Doctor has ended (but there are new ones still to be had). Sarah Jane provides the POV, and it's one full of insight and clarity into the Doctor and the life and choices of a companion; it's sad and hopeful and ultimately so rewarding to read. The story has a dreamy, poetic tone, and the two listed relationships are written truthfully and realistically.
The ultimate crackfic. Anything that can be summed up as "two Time Lords, a string of dead celebrities, and some snakes on a plane" clearly is win, of course, and the story more than lives up to the summary. We get a Doctor who doesn't give second chances and names his pet snake, and a Romana who has her obligatory Time War-related Emo Moment (tm), but also enjoys shagging her way through dodgy celebrities far more than you would have expected. It's absolutely hilarious, and manages to skewer more than a few clichés along the way. (And as a bonus, there's a short sequel!)
Captain Jack. Meeting (and sometimes shagging) his way through the DW canon. A tremendous amount of fun, and even if you don't know each character involved (I still haven't read any Fitz, myself) there's bound to be some you know and love. Some gorgeous characterizations - “It’s not as if I expected to walk into your—invisible spaceship, now, is it?” Harry retorted, nettled. “Now, look here, old chap, you can’t just leave things like this lying about where anyone could trip over them.” - a mix of funny, insightful, and hot scenes, and a moment in UNIT's history that will keep me amused for the rest my life just picturing it all add up to an incredibly enjoyable fic.