Zechs/Noin is the only one that really interests me, even though she really could do better. Duo/Hilde is cute, but not around much.

I'd say Heero/Relena was pretty obvious, especially on her part, but the idea of it always struck me as mutually unhealthy. (Though I suspect I'm the only GW fan who finds Heero entertaining, but isn't very interested in him.) I think the creators were pushing the idea pretty hard, but never wanted to bluntly admit to it.