Gmail should be available by default on Android, installable from iOS, has great web client with many plugins to get notified when new mails arrive. Always up to date and easy to install.
Pro Sleek interface
The application follows Material Design guidelines with bright, flat colors. It consists of three main views - the main view display recent messages grouped by conversations, the message view that show's threaded messages and a hamburger menu that can be accessed with a swipe from the left side or by clicking the hamburger button at the top of the main view allows switching between various folders and accounts.
Pro Tabs to automatically organize your e-mail
Gmail offers the ability to automatically organize your mail into specific categories:
Each of the categories can be disabled and you can move your mail around from category to category or set your own rules which sender should be put in which category.
Pro Incredibly fast, advanced & predictive search
Gmail brings much of the functionality and power of Google's search engine to the app's search. You can use operators such as label:, is:, OR, has: among others, define date ranges, limit search to specific groups and so on. And, you can do it all amazingly fast. The search will also try to predict what you're trying to search for before you've even finished your query and offer up those results.
Pro Google Drive integration
The app allows inserting files from Drive straight to email or uploading attachments to Drive before sending them out. It will also notify if the person you're sending the file can open the file our should you change the sharing settings for that file beforehand. Finally, you can choose which one of your Google accounts you want visible in the app.