Let me say finally why I think Heero and Relena belong together. I think, that the show makes it very, very, blatently obvious that Heero is head over heels in love with Relena. He thinks about her constantly, he always saves her from her death, and his is unable to kill her every time he has a chance. He even asks himself "What is wrong with me?" when he saves Relena during the early episodes. Heero gives Relena a birthday present in the last episode, and smiles. At Relena. And, the fact that Heero still protects Relena after Endless Waltz is a given. Why would he watch over her if he hated her? It doesn't add up. In the manga, Blind Target, the author says something like; "Relena turns to Heero for protection, and he for comfort. But don't expect him to be admitting Relena his feelings anytime soon." We all know that Heero hides his emotions, even though he said the best way to live your life is to act on your emotions. Again, I'm not saying that 'I hate yaoi' or 'I'm against yaoi in Gundam Wing', I'm just saying my opinion. I must admit, I've made fanfiction of 1x2 before, it doesn't mean that I hate one or the other; it just means that I'm open-minded.
Duo is extremely protective of Hilde, whenever she's in trouble, he saves her, and doesn't want her to fight. When Duo notices that Hilde risked her life to give her the data disk, he gets angry at her for getting hurt for him. And notice how he complied when Hilde was threated to be killed. After Endless Waltz, they work together at a junkyard!
As for Trowa x Quatre, i just think they are just friends. Have you ever heard of close friends? It doesn't mean they're gay just because they're friends. I mean, come on. Again, I'm not saying I'm against it, but I have never seen proof that they are lovers. This falls into the 'Perfect Soldier' catagory on why Trowa seems to 'love' no one except Catherine. I'm going to put a page on why I think Trowa is the perfect soldier, and not Heero. Which partially fits in this catagory, because, in order to be a perfect soldier, one must have no distractions from the mission. Even if that means being cruel to the ones you love. Fortunatly, Trowa never gets distracted, he is practical, logical, and cunning. I find Heero misses some of these traits, and he makes too many assumptions. He also gets involved into things unecessary to the mission (i.e. protecting Relena, fighting with Zechs), where Trowa does not.
Trowa Barton is probably the most mysterious pilot in GW. Who knows if he loves anyone at all? Trowa seldom speaks and when he does, he speaks truth, wisdom, and insights. He has an emotionless mask, not unlike Heero, but different. Trowa does show that he cares about people (i.e. when he saved Quatre, and when he told Heero that, "Her tears stopped me." about Catherine). Some people write fics about Trowa x Relena, and I can say it's possible. Heero is in a sense more cold than Trowa, and Wufei is even more cold and cynical. Therefore, Trowa, the enigma; who knows if he has a secret love?
Duo Maxwell x Hilde Schbeiker - I think DMxHS are a perfect couple. I don't believe in 1x2, 2x5, or any yaoi pairings involving Duo Maxwell.
It seems like every pilot has a female partner, but I think Hilde and Duo are one of the best couples in GW. Why? Because they are very similar in personality. They're both stubborn, cheerful, willing to die for their beliefs, and they risked their lives for eachother. Hilde risked her life giving Duo the data disk, and Duo risked his life saving her whenever she was in trouble. The first time they met, they acted sort of hostile to eachother, but none were willing to kill one another. Hilde forsakens all she's ever believed in just when listening to Duo talk about his beliefs. She betrayed Oz and risked her life just to help Duo out on his missions. She lets Duo stay at her house, even though she knows it's trouble for her.
Trowa Barton x ? - I do not believe in Trowa Barton x Catherine Bloom OR 3x4 pairings.
Although I do admit Trowa and Catherine have a close relationship, I think it's more of a brotherly, sisterly kind of love. There is also some speculation that they are related because of the Episode Zero manga, but since this manga isn't official, it hasn't been proven for sure.