Plateau

a Mountain enters the battlefield — 64 cards in the game can cause this (Plateau is one).

1. Last Light of Durin's Day — When Plateau enters, Last Light of Durin's Day puts a quest counter on itself
2. Staff of the Flame Magus — When Plateau enters, Staff of the Flame Magus gains you 1 life
3. Valakut, the Molten Pinnacle — When Plateau enters, Valakut, the Molten Pinnacle deals 3 damage

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

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

a land enters the battlefield — 1,542 cards in the game can cause this (Plateau is one).

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

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