Having travelled to China a few times, I found these apps to be pretty useful when travelling here:
ExpressVPN: this is essential. China's internet is sealed off from the outside world and you'll need a VPN to access Facebook, Instagram, Twitter, YouTube, CNN, Google, Gmail, etc. (Reddit works without VPN for some reason though). Get this app for both your phone and computer. Keep in mind that even with VPN, these websites will be really slow to load.
Navigation apps:
Baidu Maps (if you read Chinese): I found that Google Maps doesn't have as much information about China compared to Baidu maps (plus Google Maps is banned so you'll need VPN to use it). Really important to have this app when you're navigating around the cities
Apple Maps (if you don't read Chinese and use iOS): it's the best option for navigation if you don't read Chinese. Includes public transportation info
MAPS.ME(if you don't read Chinese and use Android): interface is a little clunky but it will provide you with decent enough navigation info. Includes public transportation info
MetroMan: this app has metro/subway maps of a lot of the major cities in China (Beijing, Shanghai, Guangzhou, etc.). You can put in the starting station and the destination station and it'll tell you exactly how to get there, which line to take to which station, how long the trip will take, how much it'll cost, etc.
Google Translate: helpful when you want to convey some complex thought. Simple sentences (like greetings) you can just remember and say. Instead of trying to pronounce complex sentence I'd just show the translated sentence to the person I'm speaking to.
Didi: China's Uber. I don't bother with regular taxis - it's hard to communicate with drivers as most drivers don't speak English. With Didi you don't have to talk to your drivers at all, everything is automated. Make sure you download this app and set it up before you go to China, as I recall part of the step involves them sending a confirmation code via SMS. You can set it up with your credit card.
Mobike: while subway is probably how you'll most likely get around Chinese cities, you may want to bike in some areas (like the french concession area in Shanghai). Mobike is one of the big bike sharing apps people use a LOT here (the other big bike sharing app is called Ofo). You can link it with your credit card for payment. The cost is really low (like a dollar an hour) and these bikes are everywhere. The thing I love about these bikes is that they're dockless - meaning you can just drop them off anywhere, rather than returning them to set docks. Again, download this and set it up before you leave for China.
Ctrip: I use them to look uphigh speed train schedules. You don't necessarily need to download the app, their English website works great as well. Make sure you don't buy rail tickets with them. I tried it once and found the whole ticket delivery process cumbersome. Much easier to just buy the tickets in person at the train station.
Alipay: this one is optional. Credit card is not very widely accepted in China, but Alipay (which is a mobile payment app) is really really widely used. Even street food vendors accept Alipay. That being said, I found that it was not an issue to get by with cash either.
WeChat: completely optional. While everyone in China uses WeChat, as travellers you don't really benefit from having it. If you plan to stay in China for an extended period of time (like more than a month), then download it, otherwise you'll be completely fine without it. Though I did run into some restaurants in Shanghai giving me discounts if I followed them on WeChat.