Mountain

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

When Mountain enters:

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

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

When Mountain enters:

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

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