The series from the Universal Century Timeline are pretty good. All those from the alternate universes though (aside from Turn A Gundam) are pretty shit imo, since they become less about actual story and the themes of a war drama and far more about merchandising and cool explosions, losing most of it's substance.
Best series of it I've seen would definitely be Gundam: The Origin. Although it suffers a bit from prequel syndrome in knowing which characters will survive, it's still a good story in seeing Char's rise to power.
As a guide for those wanting to get into the series, the best way to watch the UC timeline main series is:
Gundam Movie Trilogy (Mobile Suit Gundam, Soldiers of Sorrow, Encounters in Space) - Takes out the needless filler of the original series and doesn't have a rushed, messy ending.
Zeta - Direct sequel to the original. Don't bother with Double Zeta though, since it's far more nonsensical and only relates to the series via name alone.
Char's Counterattack - Not the best movie of the franchise, but a good climax to the main conflict of the UC timeline.
Gundam: The Origin - The anime and manga are unfinished as of yet, but it is a must watch for those who like the original.
The UC spinoffs worth watching include War in the Pocket, 08th MS Team and Thunderbolt.
