In Nigeria today, we do have so many Internet Service Providers. It will be wrong for us to compare Tier 2 networks with Tier 3 Networks. Ex. Comparing MainOne (Tier 2) with Spectranet 4G LTE (Tier 3) (Though Mainone is a Tier 2 as well as a Tier 3 Network). They do have their major differences and different audiences. You do not expect a multinational Company to depend on 4G LTE dongles for Internet, They would definitely go for a Fixed dedicated Internet. Having made some clarifications above and based on factors like Speed, Link Deployment, Coverage and Customer Service Convenience.
Here's how i Would Rank the Top Best Internet Service Providers in Nigeria
Internet Service Providers (Tier 3)
BACKBONE INTERNET PROVIDER (Tier 2 Networks)
ACE (Africa Coast to Europe)
Glo SUB MARINE CABLES
NB: It should be noted that this aren't the only internet service provider In Nigeria, There are quite a number of them In Nigeria. The speed of all networks are dependents on so many factors such as your closeness to their base stations, Mode of connectivity, etc