Tell us your favorite on-screen couple, whether canon or not. I'll start:
1. Sam/Dean (Supernatural): The supernatural brothers. "What?" I can hear you screaming, but they're the most popular pairing on fanfiction.net, followed by Naruto/Sasuke and Harry/Voldemort.
2. Barca/Auctus (Spartacus: Gods of the Arena): I just love them, their synergy and playfulness. Brutal in the arena, tender in bed. I never liked Pietros, he was too "weak" for my tastes.
3. Agron/Nasir (Spartacus: Vengeance): Can someone say "cute!" They just click, the gladiator and the "wild dog."
4. Harry/Voldemort (Harry Potter): There's just something about the hero and the villain engaged in a secret, torrid love affair. Especially Dark Harry.
5. Booth/Zack (Bones): I just love 'em, sue me.
6. Sam/Lucifer (Supernatural): Sacre bleu! Nothing else need be said. It's tres hot!
7. Dean/Castiel (Supernatural): If you look close enough, you just might see it in seasons 5 and 6. Afterall, Castiel "disobeyed for Dean!"
8. Loki/Thor (Thor, The Avengers): The angst alone... the Thorki genre is the most patronised on fanfiction.net.
9. Loki/Ironman (The Avengers): FrostIron, aahhh...my favourite Avengers pairing.
10. Bruce/Stark (The Avengers): The supernerd and the billionaire playboy philanthropist.
_
These are it, for now. List yours.

Oh and before I forget:
11. The Doctor/The Master (Doctor Who): Two super-powered time lords with obvious history. Hmmm...
12. Jack Harkness/Ianto Jones (Torchwood): It's canon, quite alright, but then they had to kill Ianto. Miss you Coffe Boy!
In fact, I'm down with any hero/villain pairing. I'm weird like that. Lol

From your list:
-Agron/Nasir
-Dean/Cas
-Sam/Dean

Elsewhere:
-Khal Drogo/Daenaerys Targaryen (Game of Thrones)
-Tyrion Lannister/Shae (GoT)

No others come to mind right now.

And these:
13. Naruto/Itachi (Naruto): The fox and the raven. Can somebody say "itadakimasu!"
14. Batman/Joker (Batman): "Why so sexy?"
15. Superman/Doomsday (Smallville): The actors are hot, sue me.
16. Superman/Brainiac (Smallville): I'm just speechless. That's how hot the pairing is.
17. Sydney/Vaughn (Alias): I don't normally like het pairings, but they've gone through so much tribulations I just had to like 'em.
18. Leonard/Sheldon (The Big B@ng Theory): Or as I like to call 'em, the hot nerd conspiracy.
19. Raj/Wolowitz (The Big B@ng Theory): they shoulda made 'em canon.
20. Chuck/Casey (Chuck): In which Chuck teaches Casey how to love. Aww...!

21. Reid/Hotch (Criminal Minds): what can I say, I like older guys.
22. Reid/Morgan (Criminal Minds): The playboy and the prettyboy.
23. Naruto/Kakashi (Naruto): The teacher and the student, the foundation of every after-school special.
24. Kobojunkie/Beaf (Nairaland): aka politically incorrect nerds.
25. Seun/Mukina (Nairaland): aka keepers of Olympus.

Oh and last but definitely not least:
26. Kurt/Karofsky (Glee): there's something about a bully and his target suddenly falling in love. Whatever, I'm a romantic, and a masochist, so sue me. Kurt/Blaine is just so...bland.

Alan Shore & Denny Crane ~ Boston Legal

And
27: Sherlock/Watson (Sherlock Reboot): Gotta love the Brits, they're less bashful about double entendrés than the yanks.
28. Sherlock/Moriarty (Sherlock Reboot): hero/villain buggering each other. Hot!
29. Merlin/Arthur (Merlin BBC): "Two sides of the same coin." "Destined for each other." If that's not the double entendré to end all double entendrés, then I don't know what is.
30. Merlin/Lancelot (Merlin BBC): Almost canon.
31. Merlin/Gwaine (Merlin BBC): Hot!

Anybody?

Julia Roberts and Hugh Grant (Anna Scott and William Thacker) in Notting Hill

Don't know if they can be considered a couple but Clint Eastwood and Hilary Swank in Million Dollar Baby.

Ramsey Noah and Nse most probably because of their chemistry in Reloaded.

Derek/ Meredith. Greys anatomy Lucas/ Peyton. One tree hill

Sam/dean (supernatural)
Lex/ottis (superman)
Mr. Smith/Mrs. Smith (mr & mrs smith)
Many more, cant remember now

32. Clark/Lex (Smallville/Superman): I've got a crush on Rosenbaum.
33. Lucas/Nathan (One Tree Hill): They're half bros so it's not really incest. Lol

I was rooting for dan/blair gossip girl (lets not even go there)
Brymes/Dominique on shipwrecked, I cannot get over that couple
and there was this couple on takemeout. Forgotten their names
Samantha/Joey The only way is essex
brb for more. . . . . .

Leonardo dicaprio nd kate winslet in 'titanic' nd 'revolutionary road',