Hmm my favourites would have to be (In no particular order)

Gundam X - Dreams by Far nothing Comes Near it...

Gundam Wing - Last Impression. Also Mission Accompished cause that song made the first ep kick ass...

Gundam Seed - The best song IMO is Tears by Nicol (also Called Nicols Tears). A close second would be River and Meteor which are by far the best tracks from GS.

GSD - My fav is "Gaia X Chaos X Abyss" and most recently Vestige (Woot for Radio Rips) but id have to say Fields of Hope is the best Destiny Song...

Its interesting cause Wing has a Victorian style distinction and Seed has a completely different sound but both suit the Anime's style brilliantly...