Australia - Land Down Under [22-3=19] People talking like it's easy to place districts on tiles with breathtaking appeal, I even saw a vote where a person said they could get a +3 Campus, +3 Theatre Square and a +3 Holy Site in every city as Australia and that it is more powerful than Japans. Every city having having 3 powerful districts from breathtaking appeal? Forget about it. Oh and you better hope there are no mines next to your breathtaking appeal tile, or that district is just gonna be turned into a plain ol' district. Unless of course you don't want to build mines, when production is the most powerful commodity in Civ VI. (you build units, wonders, district buildings, Spaceship parts quicker, etc.) PS: It appears "Land Down Under" is highly incompatible with a culture victory. I've seen people say why go culture when playing as Australia? If a Civilization ability stops you from pursuing a victory path, that seems pretty horrid.
Greece - Plato's Republic [24]
Japan - Meiji Restoration [18+1=19] Playing as Australia your districts get +3 adjacency when placed on tiles with breathtaking appeal? How about you play as Japan and place a district triangle of city centre, campus and theatre square. There is your +3 campus and +3 theatre square which you can place in every city, plus you can place mines without the district becoming a bog standard one. You also get to keep your National Parks and Seaside Resort tiles spots so that is more tourism on top of the top notch district adjacencies, whereas Australia gets less tourism. How about you plan ahead, find some flatland between 2 cities and put a government plaza in the middle of a pack of districts? You can get more than double the adjacency bonuses of Australia.
Kongo - Nkisi [14]
Netherlands - Grote Rivieren [15]
Nubia - Ta-Seti [24]