Google Translateworks offline
Best overall translation app
If you were to only use one translation app, it'd be Google Translate. It supports the most languages (103), lets you download most for usage offline, and is now more accurate than ever thanks to AI. You can translate through five methods: type, speak, draw, picture, and live video translation (which can be really useful for signs).
Sorry iOS users, but Android gets a special "tap to translate" feature that lets you translate directly in chat apps. If you've ever had to chat in a foreign language, you know how big of a time saver that could be.
Microsoft Translatorworks offline
Great alternative, especially for live conversation translations
Microsoft Translator only supports 60 languages to Google's 103, but if you were to look beyond that one measure it's actually better in a few ways. Google Translate lets you save phrases for future use, but Microsoft goes further and includes an entire set of commonly used phrases for quick reference. Microsoft's image translation is also easier to use (you don't need to manually highlight words).
However, the biggest feature that sets Microsoft Translator apart from Google Translate is simply called "conversations" that translates conversations in real time between multiple people and devices. It lets people partake in discussions regardless of language in a more natural way vs. having to constantly speak or type into a phone in a jilted back and forth manner. It's what a future where language isn't a barrier would feel like.
Baidu Translateworks offline
Best for Chinese or when in China
We recommend Baidu Translate if you're going to China (since Google is blocked there) or if you want more accurate Chinese translations like local lingo. Baidu is basically the Google of China so it's not surprising Chinese is their specialty.