What do you believe are applications that an enterprise should always buy instead of bothering to build?

ERP (and keep configuration as standard as possible)
CRM
And last Email. (And for email use hosted services like Microsoft or Google)

ERP

I think it's usually depending on the budget and resources available, it may make more sense for an enterprise to purchase applications rather than trying to build them in house. Generally speaking, applications that require complex algorithms such as those related to big data and artificial intelligence are more suited to commercial off-the-shelf software than building something in-house. Also applications that need to integrate with third-party APIs are also often more efficient to purchase rather than build.

Any application where you don't already have the skills on hand to develop/support/maintain.