Excellent question, which as usual does not have a simplistic answer. There are Global Tier 1 providers, such as Zayo, NTT, AT&T. There are partial Global Tier 1 providers, such as Vodafone, Liberty Global and Telia. The list is not exhaustive.
In the UK I’ll refer to Tier 1 providers who provide Ethernet Fibre, not to be confused with fibre broadband, to the site or premise. UK Tier 1 leased line providers include: BT, Vodafone, Colt, Virgin Media and SSE along with providers, such as KCOM, with legacy presence in specific areas.
National Tier 2 and Tier 3 providers can be described as either having to use a Tier 1 or Tier 2 for some portion of the leased line, or they have a contractual arrangement which places them in Tier 2 or Tier 3. A Tier 2 example would be ourselves, where we have a leased line provided end-to-end by a Tier 1.