UC OVAs (should be watched in order)

• The 08th MS Team
• 0080: War in the Pocket

AUs (can be watched in any order)

• Wing
• Turn A
• Seed + Seed Destiny
• 00 + 00 Second Season

Sources:

• Where to start?
• Sir Haggis' Guide to Gundam
• The Gundam Multiverse is Huge and Confusing - And We're Here to Help
• Quante e quali sono le serie di Gundam?