For the purposes of this post, I will use the following definitions of the tiers.
- A tier 1 ISP is one that relies purely on peering and does not use transit.
- A tier 2 network (may or may not be an ISP) is one that uses a mixture of peering and transit.
- A tier 3 network (may or may not be an ISP) is one that relies purely on transit.
You can't go pointing a default route at a peer, at least not if you want them to remain a peer in good standing. So the core routers of a tier 1 ISP will need to be default-free. To reach the whole internet without a default route, a full internet routing table is needed. Therefore the routers in the core network of a tier 1 ISP will need to carry the whole internet routing table.
A tier 2 or tier 3 network on the other hand can choose if they wish to filter the routing tables from their upstreams and peers and use a default route pointing at one of their transit providers for the residual traffic. This saves them router resources, but has the downside that if their upstream provider gets in a peering dispute they may lose access to parts of the Internet.
Many ISPs will split their network into "core" and "access" portions. The "core" side will have a full internet routing table, while the access side will have default routes pointing at the core side.