As people have said, along the coast, near lots of mountains, along river even, and with forests if possible.
If you want the BEST, I'd also suggest: the city itself be on a flat tile (no hill) for a Windmill, and that it be near a Desert tile maybe (for Solar power eventually?), Hydroelectric gets 1 hammer for each tile next to a river. If you're worried about having a desert tile being a waste, you could drop a Great Engineer on it to create a factory or something.
On the coast, you can get three hammers from Commerce social policies, and you can get 5 hammers in all cities from Order social policy. And another hammer from Liberty social policies. Those nine hammers are huge.
Since you're on the coast anyway, I would suggest also finding a spot that has 2-3 (or even more!) sea resources (fish, whales, pearls), because Seaport will give you +2 hammers for each sea resource, and sea resources can also provide a lot of food which you aren't likely to get from your land tiles. If it's late game, you can even get offshore oil platforms, which add even MORE hammers.
As people said, there's a lot of different factors that can go into it. I don't look for all these things, I was just trying to design "the most hammers possible." Obviously, food is important as well, depending on how you use maritime city states especially.
edit: Wu Zetian, is that on Legendary start? I've never seen such resources!