Urborg

a land enters the battlefield — 1,560 cards in the game can cause this (Urborg is one).

1. Primeval Bounty — When Urborg enters, Primeval Bounty gains you 3 life
2. Toph, Earthbending Master — When Urborg enters, Toph, Earthbending Master puts a counter on a permanent
3. Dancing from Dark to Dawn — When Urborg enters, Dancing from Dark to Dawn makes a token
4. Emeria Shepherd — When Urborg enters, Emeria Shepherd brings a card back
5. Quest for the Necropolis — When Urborg enters, Quest for the Necropolis puts a quest counter on itself
6. Soul Stair Expedition — When Urborg enters, Soul Stair Expedition puts a quest counter on itself
7. Nissa of Shadowed Boughs — When Urborg enters, Nissa of Shadowed Boughs puts counters on a permanent
8. Thranduil, Sindarin Liege // Silvan Rally — When Urborg enters, Thranduil, Sindarin Liege // Silvan Rally makes a token
9. Silvan Reveler — When Urborg enters, Silvan Reveler returns itself from the graveyard
10. Floral Evoker — When Urborg enters, Floral Evoker puts a +1/+1 counter on itself

199 other cards care about it too. These are the ones with the most connections.