Well, to be less short with the answer, let's look at the pros and cons of each.
ATLANTA
Information in the Neo-Anarchit's Guide to North America, Shadows Across North America, and Dirty Tricks. This is a well-covered city with tons of plot hooks. It's in North America, so Seattle-ish, but not Seattle, so gets to be different. You mostly have English speakers and a familiar-ish culture. Lots of major players are active here including the HQ of Saeder-Krupp Prime's North America branch. You ALSO have the CAS federal government (and the state and the city) here and major news organizations, so, crawling with game options.
SYDNEY
Information in Target: Awakened Lands, the Sixth World Almanac, and I *think* one other book but not sure which. (Cyberpirates maybe?) so covered, if not as well as Atlanta. Still a majority-English speaking area but with more options than the CAS would have due to the Pacific and Asia. Far less play from the major corps but, with smaller levels of corp activity, the PCs are more influential and can cause significant damage instead of being a minor nuisance, With fewer runners around, a smaller support network and less to hide behind, so, good, bad, you take the gamble. Lots more magic than the other options.
BERLIN
Covered in the Germany sourcebook, Sixth World Almanac, and Shadows Across Europe, plus extra coverage in the assorted Corporate books. If you speak German, you have a MASSIVE number of options from the Pegasus crew. The city with the most one-party control of any, everything is going to revolve around S-K here. This will limit your storytelling somewhat, but allow for more depth than breadth which isn't necessarily a bad thing. Less diversity in terms of language, but more Metahuman presence than average and less oppressive than, say, Atlanta would be.
MOSCOW
Covered in Target: Awakened Lands, the Sixth World Almanac, and Shadows Across Asia, this one's got the least coverage of any, so more room to make it your own. Corporate presence isn't dominated by any one side, but the organized crime levels are off the char with the Vory. You have access to the Russian government and military (A big issue since the Russian invasion kicked off the Euro Wars and there's ongoing Russia-Poland military tension) and a society that's not so big on the outlawing of things, so expect PCs to walk around with brazen levels of firepower that you wouldn't get elsewhere. Magic will be more unusual and Shapeshifters are very much a Thing.
From there, you'd want to decide what location fits the stories that you want to tell the best. Traditional Shadowrunning? Atlanta. A bigger focus on wild magic and natural spirits? Sydney or Moscow. High-end Black Trenchcoat work? Berlin.
Ultimately, your call to make.
