question

I would advise you to use SQL Server 2016 Standard SP1. This is the latest version. You can only purchase the latest version, (but run earlier versions in a downgrade scenario) so why not have the ability use the all the new features? Whatever version you choose, make sure that you check for any deprecated features that you might be using in SQL 2008 R2. https://msdn.microsoft.com/en-us/library/bb677622.aspx

The version you will need to upgrade to is one that all the connecting application support. Typically you can only purchase the latest version from MS (currently 2016), but they are backwards compatible (always check with a licensing expert - as I am not). Contact all your vendors to see if there application support 2012/2014/2016, as that may make your decision easy. That said, if you go with SQL 2016 SP1 then Microsoft have opened up a number of what used to be enterprise features (see [here][1]) in the standard edition. [1]: https://blogs.msdn.microsoft.com/sqlreleaseservices/sql-server-2016-service-pack-1-sp1-released/

As new features added to SQL Server 2016. So its good to use updated features. Here, you can see how to Upgrade to [SQL Server 2016 in an Effective Way][1]: [1]: http://www.sqlmvp.org/upgrade-to-sql-server-2016/

I agree with the above two comments (go for SQL 2016) but consider the following points make sure that if any of the application/s that you are supporting is compatibility preference.