In the past I was always told by colleagues 4.2.2.2 appears to work faster and have to combine that with Google's public DNS*. Using 8.8.8.8 as primary and 4.2.2.2 as secondary. However, Google only provides 8.8.8.8 and 8.8.4.4, so who provides 4.2.2.2? And if true, why is 4.2.2.2 faster?
*Note: These IP addresses are used to dig domains and to determine if local DNS changes has propagated over all over the internet.
4.2.2.2 is one of six (4.2.2.1 through 4.2.2.6) DNS servers run by Level 3 Communications, a Tier 1 ISP. These are supposed to be used by Level 3 customers only, although they have gone into general use over the years. Supposedly, they are officially discouraging people from using these DNS servers, as they are under no obligation to provide this service and can theoretically discontinue them at any time (whereas Google provides their DNS servers expressly for public use).
More information here.
