OTP: I'm going to redefine the common meaning of OTP and have it mean Original True Pairing for a moment and name Luke/Leia my OTP. Yes, I know, in a post-May 1980/1983 era that sounds pretty disgusting, but since I like to consider myself part of the OT's prime target audience, I fully accepted that the Hero and Princess would go on to live happily ever after. While I wound up becoming a big fan of The Scoudral who would eventually wind-up getting The Girl, I still liked to think I had more in common with The Hero and I felt sorry for Luke when he didn't get The Girl in the end.
Now, back to the original definition of OTP. My One True Pairing will have to be Luke and Mara. Finally, The Hero gets The Girl and she's a feisty one, wasn't she? Not only that, but she hated his guts when they first met, too (a bit like Han and Leia, don't you think?). Eventually, Mara would come around from wanting to kill Our Hero to wanting to marry him. That farmboy must have had one Force-ful personality.
Runner-up: Will have to be the original romantic pairing, Han and Leia.
Honorable mention(s): Anakin and Padmé. Yes, the movies didn't really convince most of us why these two would get together, but love doesn't just mean never having to say you're sorry, it also means never having to say wha? how? why???
Artoo and Threepio. 'Nuff said.
Crack pairings: Ahsoka/Rex. Hey, the age difference between these two is probably the same as the difference between Padmé and Anakin, and the female in this pair is the elder of the two, too.
Also, Winter and Lando. Admittedly, this only exists in a couple of pieces of fan-fiction I've written and have yet to write/finish, but I still think they're a good pair.
Ship everyone else seems to like, but I don't: This is mainly an EU, beyond ROTJ pairing, but I never found the Jaina/Jag pairing all that convincing, mainly because I never found Jaina to be all that appealing. Now, I stopped reading post-ROTJ EU after the New Jedi Order, so I really don't know much about what she's like now, but I'm not exactly pleased that she's the last Solo off-spring.