Is ITIL Best Practice or Good Practice?
By definition, ITIL is a set of best practices (refer to glossary and section 1.2.3 of any of the books) It is also considered a "source" of good practice. While this may be confusing, it is important to understand the distinction.
There are many sources of good practice but not all of those sources are validated as "best" practice. While the term is loosely used, best practices should be repeatedly proven to demonstrate tangible value in actual organizations. In fact, today's documented best practice could be tomorrow's good or common practice. That's how service management evolves, improves and becomes institutionalized. Building a service management program can also involve other sources of good practice (i.e., other frameworks, standards, and proprietary knowledge).
ITIL makes it clear that it's best-practice guidelines are not intended to be prescriptive. Each organization is unique and must 'adapt and adopt' the guidance. This will involve taking into account the organization's size, skills/resources, culture, funding, priorities and existing ITSM maturity. The guidance can then be modified as appropriate to suit the organization's needs. I do not know of an organization that has adopted "pure" ITIL best practices. The end result will be a set of practices that are "good" for the organization given its level of maturity.