So far I've seen 4 gundam series: Wing, Seed, Seed Destiny and OO both seasons.
Gundam Seed series are awesome to me. To watch them is a must. Wing eh... not that much and OO it's more about the amazing graphics, effects and animations than about anything else... But it's something you enjoy watching.
The Seed series have something that got my attention. More than the real cool battles and weapons and gundams and stuff, the story is very well made; consistent, beautiful and
inspiring. The series also emphazises on char's determination to fight for what they believe and we can watch the chars grow as human beings, becoming stronger when taking hard decisions and never giving up... All of that without plotless episodes.
Also I think that at least seed & OO series are completely fillerless, (except for the resume episodes in Seed)... So you'l be advancing on the story every single episode and want to know what happens next in almost every single episode as well ;P
So I think you'll be captivated by that gundam series... and for OO, You'll just enjoy watching it. Even if it has some big issues as a series, it's cool.
Donk, about Sailor Moon: