Salesforce provides a great standard UI experience that is highly customizable and adaptable to new features of the platform without you necessarily releasing new revisions of your application. At its core, it is a data-centric user experience, which means that most tasks come down to creating, editing, or deleting records of some kind. Having a strong focus on your Domain layer code ensures that it protects the data integrity of your application.

If you want to express a more complex process or a series of tasks, Visualforce and Lightning allows you to be more expressive using components or other HTML libraries to create the user experience needed for the task at hand. While this is very powerful, it is important to always consider the standard UI, and, wherever possible, augment or complement it with the Visualforce page and/or Lightning components rather than making this the only way of interacting with your application.

Equally important is to observe best practices around engineering custom UIs for performance and security. Consider carefully and choose wisely the degree to which you depart from the standard UI and/or standard components, as the further you move away from these, the more responsibility and complexity you have to deal with as a developer. Look for opportunities to create hybrid standard UI and custom UI combinations to get the best of both worlds.

Reporting can be a personal thing for your users; the Salesforce Analytics API allows you to harness the power of the Salesforce Report Designer and engine with a more tailored rendering of the reporting data returned.

At present, there are numerous combinations of technologies and approaches to build a mobile user experience for your users. As with the standard UI in the browser, start with what Salesforce provides you and your users with out of the box as standard. The Salesforce1 Mobile application is effectively a platform feature that users expect to be able to use with your application objects like any other.

Building custom UIs for use within Salesforce1 mobile is best accomplished using Lightning Components. However, if you choose to build your own mobile application, you can choose to develop it using one of the Salesforces Mobile SDK's as a starting point; alternatively, if you have the skills and experience, use them as a guide on how to call the various Salesforce API's.