Ooh, good idea!
1. Damon/Elena (The Vampire Diaries)
2. Spike/Buffy (Buffy the Vampire Slayer)
3. Castle/Beckett (Castle)
4. Peter/Olivia (Fringe)
5. Cappie/Casey (Greek)
6. Lucas/Peyton (One Tree Hill)
7. Brooke/Lucas (One Tree Hill)
8. Dan/Blair (Gossip Girl)
9. Buffy/Angel (Buffy the Vampire Slayer)
10. Logan/Veronica (Veronica Mars)
11. Nathan/Haley (One Tree Hill)
12. Doctor/Rose (Doctor Who)
13. Rory/Amy (Doctor Who)
14. Max/Liz (Roswell)
15. Nick/Jess (New Girl)
16. Stiles/Lydia (Teen Wolf)
17. Cook/Effy (Skins)
18. Jake/Peyton (One Tree Hill)
19. Barney/Robin (HIMYM)
20. Lily/Marshall (HIMYM)
If we're including movie couples, the list might be slightly different. Damon/Elena is my ultimate OTP forever, and my other top 4 shift around in their spots. The last 15 are pretty arbitrary. I haven't even finished HIMYM yet, but I'm guessing that those two couples are going to be in my top 20 list for sure.
