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The original series is definately one of the must watches, but since you've posted that as your pic I assume you're planning to watch it or at least the trilogy movies.
Zeta is also a must watch. Despite a lot of silly or just plain ugly grunt designs, Zeta is something all Gundam fans must see because it has some of the best character development in the franchise along with adding a lot to the 'real robot' genre as a whole.
Then there's 0080, which shows a more tragic side to the OYW. It's short so I can't really say much about it other than it's a must watch.
08th MS Team is a side-story of the One Year War, showing us more of a frontline view of what it was like.
If you prefer production order, then go by the above list. If you prefer chronological order, watch the original series, 08th, 0080, then Zeta.
And then there's Turn A. I fully recommend watching Turn A last after you've seen most if not all available Gundam series. It's one of the best Gundam works that is set in the distant future and ties most Gundam series together as a distant finale.
