You SeiyaxUsa fans should consider this a gift from me because you won't get a lot of shipping of them out of me!
I am devoutly UsaxMamo, but I just LOVE Seiya's honest affection for Usagi and his (or her!) true spirit. It's clear that Usagi has feelings for Seiya, but in the end her love for Mamoru is just so much stronger. But Seiya is always there for her and always willing to love her even knowing that she can't love him (or her!) back in the way Seiya wants. It's a heart breaking story, but it's a really good one!
[link] <- Skip to 20:00; one of the best scenes in all of Sailormoon IMO (episode 194, rooftop scene).