>>34395089Depends on what you're getting into. All the AU Gundams are pretty self-contained. You can just pick a series and watch it all the way through. There are obvious exceptions like not watching the 00 movie before finishing the 00 series and not watching SEED Destiny before SEED but for the most part you can pick one and watch it.
For UC, you need to establish a base understanding of the universe to get the most out of most of the series. For that reason, you should start with the original Mobile Suit Gundam, then go into Mobile Suit Zeta Gundam, then ZZ Gundam and finish off with Char's Counterattack. After you've follow this progression, you can pick up any UC series and be able to fully appreciate what you're seeing.
