Plains

a Plains enters the battlefield — 72 cards in the game can cause this (Plains is one).

1. Koth of the Homestead — When Plains enters, Koth of the Homestead puts a counter on a creature
2. Staff of the Sun Magus — When Plains enters, Staff of the Sun Magus gains you 1 life

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

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