Hmm, I'm going to be a total pimping whore and say my top dirtiest would probably be, in no particular order,
Proxy, VM, Logan/Lilly, Logan/Veronica, For Lilly Kane's fuckedupness http://svilleficrecs.livejournal.com/161499.html

Petals, Doctor Who, Ten/Rose, for drugged, public sex
http://svilleficrecs.livejournal.com/324403.html

Allow, Mac Commercials, Mac/PC Guardian, for various kinks
http://svilleficrecs.livejournal.com/468553.html

The Things You Know About a Person, Sarah Connor Chronicles, Derek/John (for dirtybadwrong, as much as the dirty)
http://svilleficrecs.livejournal.com/721843.html

A Small and 100% Innocent Token of Gratitude, Iron Man, Tony/Pepper (for foot kink and wall fucking - er, against, not gloryhole style)
http://svilleficrecs.livejournal.com/741708.html

Irreparable, Iron Man, Tony/Pepper (for sheer volume of kink and fuckedupness)
http://svilleficrecs.livejournal.com/759911.html

As for other people, the following may contain various levels of quality in other areas, but for sheer dirtybadhotness

Xmen (?comicverse) Wolverine/Gambit dubcon http://members.tripod.com/mvslash/o/experiment.htm

SPN, Sam/Jess, pussy eating
http://mona1347.livejournal.com/193946.html

SPN, Sam/Jess, pegging
http://mickeym.livejournal.com/887326.html

Iron Man, Tony/Pepper, fucking by proxy
http://atrata.livejournal.com/137574.html

Secretary, Gray/Holloway, Electricity play
http://meadowlion.livejournal.com/65443.html

Top of the List, SGA, Rough Sex
http://www.helenish.net/top.shtml

Due South, Fraser/Ray K, bondage
http://hos.slashcity.com/iaitw.htm